This poem says to not give your everything to certain people. Many of them are always changing and once they do, it can hurt you. It tries to explain to not get too comfortable with some people since the ending result can leave you with a painful memory.
Don't tell me to be happy when you're the only thing that can do that. I have everything else. You are the only thing that matters to me anymore. I'd give it all up in a heartbeat just to see you because screw all this I'm not happy without you