Why does my friend always do this? How can I get him to stop?

Every time I like a girl he'll go in to long rants about something that makes her undesirable in his eyes, or something about her he thinks is ugly, almost as if he wants me to start hating her. It won't ever change how I look at her, since I already noticed these things and accepted them, but they ruin my good mood and make me feel down. Is he jealous or something that the girl gets more of my attention than him? Also, if it matters, he's never had a girlfriend. How can I tell him to stop without seeming as if I just want to ignore the girl's flaws?