Do you deliberately have to make your character flawed in personality in some way?

Depends on how often spiders show up, I guess. In the case of this particular campaign - Horde of the Dragon Queen/Rise of Tiamat, if you're curious - there were only four major incidents:

1) 1st level, had to go through a tunnel full of cobwebs. Iliira - who prior to that point had been point man do to being a rogue - refused to go through until the rest of the party did and confirmed that there were no spiders.

2) Whilst spending 2 in-game months (and like 6 sessions) on a caravan traveling from Baldur's Gate to Waterdeep, the caravan came under attack by ettercaps and giant spiders. Iliira ran from the spiders but ended up staring at ettercaps, who were just humanoid enough (according to a self-imposed Intelligence check) to her that she could fight them. Although "fighting" in this case mostly amounted to screaming, crying, and flailing her rapier and dagger around. This was actually significant because it happened during the day, which Iliira normally slept through while on the caravan since she was on the night shift - and it revealed that she was a drow to the whole caravan (previously she had disguised herself, so only the party knew). So there was a nice bit of role-playing afterwards as various NPCs that Iliira had interacted with positively (while in disguise) came to her defense, while others wanted her out of the caravan, etc. Ye Obligatory Drow Reveal scene, basically.

3) During some quest or other in a castle, encountered a room full of giant centipedes. Turns out those freaked Iliira out as much as spiders, so she spent the whole fight pressed against a wall.

4) The forest I mentioned.

I'm probably making things sound worse than they were - for the most part, Iliira's arachnophobia was actually played for laughs. It's just that it had an actual, definite mechanical impact on her character as well. It also made her feel like a more complete character,as opposed to a piece of paper with numbers on it.

Anything where a player willingly backs out of a combat encounter sounds like a pain in the ass unless the fight is super easy.

To each their own but I would be annoyed as fuck if I had to play with a PC like that.

Again, it wasn't "had". The DM didn't impose or require flaws or anything, I personally chose going in to the campaign that I was going to play Iliira as an arachnophobic drow. In-character, because when she was four she woke up to a big spider sitting on her pillow staring at her and it freaked her right the fuck out.

Out-of-character, it's because I personally find the idea of an arachnophobic drow to be funny.

None of the fights she missed out on were difficult by any means. The only one that nearly caused an actual problem was her refusal to go through the spider-infested part of the forest on the way to the green dragon, but there were actually several different solutions to that problem.

For example, by that point Iliira had two bags of holding. She probably could have been convinced to climb into one and just be carried through. Another solution might have been setting the webs on fire (this was, in fact, one that Iliira herself suggested. For some reason no one wanted to risk burning down the forest). Then there was the Suggestion possibility; or else casting Fly (as by that point the warlock had access to it) and soaring over the area.

It was just another challenge to be overcome. Such is the life of an ADVENTURER!

It's a point against flaws always being good. Flaws are like salt. They need to be subtle to add depth and highlight the flavors to a dish. A heavy hand with salt makes a disgusting meal.

The wolf druid is a "flaw" in the same way just serving salt alone as a meal works.

Flaws can be character building and help the DM achieve greater verisimilitude.

My current character is a generally good, optimistic person, but he errs in the side of caution when dealing with certain people due to his nationalistic upbringing and the long state of intermittently cold-hot war between his country and theirs.

This kind of thing only enhances the experience. It means that my fellow party members aren't just interacting with Generic D&D Sorcerer, they're interacting Jyrem Woadrenne, who is a character in the world they're playing in and whose actions are informed by this.

>Again, it wasn't "had". The DM didn't impose or require flaws or anything, I personally chose going in to the campaign

You misunderstood me:
If I had to be in a party alongside that character (not ME HAVING TO PLAY the silly bitch) I wouldn't have any additional fun with the game due to her flaw, I wouldn't find it particularly interesting, I wouldn't find it funny, and I sure as hell wouldn't think it added any depth.

Indiana Jones had a fear of snakes but it never fucked him or his companions all that badly, it wasn't central to his character, and it was funny because his reaction was often "shit not this again" but not being paralyzed into complete inaction.

What you did was took a flaw that COULD HAVE been potentially entertaining that jumped straight over the line that made it downright annoying.

*then jumped

Indy doesn't like snakes and is sort of afraid of them, but he doesn't actually have ophidiophobia - dread when even thinking about snakes, and panic attacks when confronted by them. There were plenty of things my character didn't *like* but she could still deal with, but I wanted to play one who had actual arachnophobia.

You might personally have not derived any fun from it, but myself and my fellow players did. Like I said, it only had any real important just four times over a campaign that took more than a year of real time to complete. Out-of-character, if I'd had reason to believe that spiders would have come up a lot in the campaign - like, for a campaign taking place in the Underdark, say Out of the Abyss - I wouldn't have played it up so much since it constantly coming up WOULD have been unfun. But as things stood I felt pretty sure that it wouldn't be a constant thing in HotDQ/RoT, so I went with it.

The arachnophobia also wasn't center to Iliira's character, either - the center of her character was what I call "megalomaniacal kleptomania", in that she didn't feel compelled to steal any random thing she came across, but when confronted with the opportunity to steal something BIG and, more importantly, get wide recognition for it, she found it very hard or often impossible to resist.

Iliira's life goal was "become known as the greatest thief who ever lived" so that when she met random people on the street they'd say "You're Iliira, the thief!" rather than "AAHHH! DROW! RUN!"

There's an easy middle ground here, when the character fails at something ask the player why they failed, the player can use that to establish a flaw.

Trying to force flaws into your character is just this reactionary fad against accidentally creating a mary sue. Plenty of entertaining characters don't have any obvious flaws at all.

I don't add them intentionally. I just start out with a premise and extrapolate a character from there. Whatever natural flaws turn up during the work up of their backstory, stay.

The dice show you you failed.
Your flaws show you potentially WHY you failed.