What causes a person to constantly "take things too personally/seriously"? How do you stop doing this?

Sounds like to me they are insecure about themselves, maybe a self esteem problem. Talk to friends, get some counseling.

I am like this all the time. For example, when i'm going out with a group of friends, and another friend says that another firned looks nice, i take that to mean that I don't look nice...haha...

I think it's caused by insecurity and low self-esteem. Actually i know this...what i don't know, and i should find out...how to stop doing it

Unhappyness in there life.. They arent were they want to be and it makes you miserable. Maybe being around people who dont understand them, but not knowing how to cut ties without hurting these people because you care about them.

The environment in which they live / have lived.

Crack some good jokes.

well seek help 1st step