As TonyKing said... But I'd rather explain some on that "complex thing"
You should really get into the depth of his depression and find out more about this problem before you can solve it, and this is done by asking your friend questions. Like these:
How exactly does it make you feel down that the girls are predictable?
What do you think would make you happy again? What is missing in your life? Can you supply it? Is there anything that could replace it?
Then you can go into some reasoning without hurting him...
Obviously it's not every girl that is the source of your problem... Perhaps it's your ego generating it. And dropping it is easier than you'd think. As you drop your ego you experience that the problem is fading away... ceasing to exist.
Okay perhaps I went little NLPish
Don't try it on a depressed friend unless you're a seriously experienced communicator... But anway... those questions can't hurt and it can really help to find causes and possible solutions to the problem. The idea is to give him choices. Or better drawn... make him give himself choices. Because currently he sees none.