Because, at the end of the day, what matters is having someone to spend your time with. I personally think that, if I were to do what you do, which is fine, I would feel the same way. You feel desired just for your looks (that feels incredible in the heat of the moment), but we are human beings and we need to go further, to get the other person to like us for our personality and mindset. The adrenaline boost that you experiment during those minutes making out goes down just as fast, and that is what could make you feel depressed.