For why I don't or do join RPs:
Do:
1. Boredom, since I have nothing else to do with my life because I'm one of those people
2. Interaction, this is what I usually do with characters, what if they were here, what if they were at some point in the past, etc. It's a way of creating alternate beings.
3. Theme, some times I just like the theme or don't. Most themes I can roll with given the amount of characters I make.
4. Early, when I see a new RP, I usually try to get in it first to kinda get an anchor into it, so I can have someway of telling what is going on or whatever I need.
Don't:
1. Pre-Existing lore. Some stuff would just conflict with my characters and just would make playing though it not fun overall. Though I can easily change it by using a different character.
2. Lack of explanation, usually this is when I just have no idea what is happening.
(Oh boi a very large reason.)
3. Continuation, I wonder if this is even a surprise to any of you, but I have a somewhat complete timeline of all my characters. (Because I'm one of those lore guys who go crazy about it.) I usually dislike using characters that I have used before without some sort of context (Reason why you don't see multiple versions of the first Teridax(Note I said first, not Makuta nor Glitched) here and there.) And if a certain roleplay takes place before or after another, it becomes weird. (Reason why with the OfficalEyeOfCthulhu's RPs, I didn't reuse Lobotomite Continuation and instead went with Traps.) There are the rare case where an RP doesn't even start, which I then make the exception to reuse characters. Also, if you want to use me using Smash Ultimate to get all my characters except for certain villain characters, all of them has some reason that they appear there. Usually involving dimensional shenanigans (Like Blade) but eh. It actually didn't conflict with any of my pre-existing lore so yay for me.
4. Self Insertion, usually this is where you play as yourself. This was the main case with Random Forum Fights, and I basically got tired of being myself. (Wow, never thought I'd say that.)
5. Too late, usually, as this is the case with most Discord RPs, I just pop in and have no clue what is happening which basically causes me to just nope out of there.
Not sure if that helps, but eh.