First of all, I would like to apologize if my question sounds ignorant or anything because I will stay ignorant forever if I don't ask this.

My question is...
how and why, someone with a perfect life and always happy can have depression? As for me, I am depressed because my life is so ******, I am useless, my existence is nothing, no one needs me, I am ugly, I am fat.....I feel all that until I thought about suicide. So...what's going through in someone who's depressed but happy's mind?

Again, I'm very sorry if this question sounds rude. All that I know is depression doesn't have a face. I just want to know what's going on their mind that bring them to being depressed, suicidal etc.