because that is the way that they feel. i don't think that they do it potentially because i'm always very depressed and i have a lot of suicidal thoughts but i think that some people can be phony and say that they are to draw attention to themselves, granet the fact it's probably not the most positive attention you could draw to yourself they still do it because they have no other way. i do think that if they are really depressed than they don't see happiness because well their depressed and because they don't see happiness all they see is sadness and anger. so i don't think that they really mean to put you in a bad mood. i think it just happens because it's they way they feel about things and it's the way they see things. i hope i helped you in understanding that.
Have you heard the saying "Misery loves company?". Don't let them bring you down! For every negative thing this person says, give a positive reply. Before long they will quit acting that way around you because they know you won't play into it.