Don't Understand

I remember times when I was so negative about myself, but I never thought to bring down other people, especially when they are already down themselves. I"m baffled with how people respond to a person who is down. It's nice seeing the positivity that people bring to them, but then you have those that make them feel worse. I was on Facebook today, browsing the usual days worth of statuses and pictures and what ever else to keep my laying in bed and procrastinating from sleep or other productive things I could be doing. I come across one of my friend's statuses saying "If im strong enough then why are these thoughts racing through my head...." What would you put? I think that most people would put something to lift the person up. Give them a reassuring way of life with positivity like "I'm here for you" or "You are strong enough" or something like that. That's not what the second comment said on his status.... it said "maybe your just not strong enough" -_-' i want to punch this person so hard right now. Would you have ever thought to put that? Would you wish to assure this person that they wouldn't be strong enough even though everyone is strong enough for what they go through? God wouldn't put us through these things if we weren't. Yes, we need to rely on Him, but I mean with just the thoughts a person can be strong enough to overcome that kind of thinking. With help, I sure did. 
What makes this worse is that this person considers himself a friend to the one who is down. They even live in the same house! I was baffled to see that. I know this is petty anger, so maybe it's other frustrations that are getting at me and this blew me way over. I just never could understand how people could do that to others. I understand being negative towards yourself, but I don't understand doing that to your own friends. Friends are meant to help, no dig the hole with you. Let a lone this person that did this wasn't the smartest and I have had a lot of frustrations with this person because he only cares for himself. When I proved him to be so, he contradicted himself. Either way, it bugs me. I have seen others do it, and I don't understand why. Why is attempting to help other people to be happy so hard? Don't people like it when others are happy? Don't people like it when they have smiles back on their face or can say thanks for being there or cheering me up? Why is that so hard to do?