My girlfriend requested that I changed my phones password, but I don't feel like I want to.
Are you serious? Why would she make you do that?
I'm not interested in knowing the reason, because if I'm interested, I'd change it. If I don't know why, then why should I change it?
It's your pin you should pick what you want. You shouldn't change it just because they want you to.
Right. I'm paranoid that they might want to look at my tinder and instagram/snapchat
If they communicate that with you, and you consent to them looking at that stuff then I guess there's no issue. But there may be some trust issues that your girlfriend has and you should do your best to alleviate those.