what goes up, must come down?

a few years ago I read a book that said that you could always think yourself happy and that happy thoughts would generate more positive things to come to you... I think of myself as a positive person and I do try to always think about the good things in life and really treasure when things go my way, but there's always something holding me back at the end. Is that coz I expect there to be something difficult waiting for me or is it just because nothing in life can just go on and on upwards?! Who knows... if life gives you a problem, you deal with it the best you can and always make the best of the good times you have coz life is too short to be depressed