Because after you reach a certain age all you're gonna truly have would be your family. Older members of your family now won't be around forever. Friends will start their own families. You need someone you love by your side as you age. There's nothing as beautiful as having people of your own to share your happiness and sorrow for the rest of your life. It's certainly not an obligation. But it's better to share whatever else you've accomplished in your life(career, wealth, fame) than swim in an ocean of money only to look around you and find noone there.