Because University was the best time in your life, and every day now is a dreary slog of mediocrity. By giving money, you can reclaim a part of your glory days that won't come again. You can feel like giving to other students will help their lives, as a pale imitation of what it would be like to be with that group again, having fun and chasing girls instead of supporting a dead-end marriage with a mind-numbing job. If you have enough money, you can even inject your name into student life by getting a building named after you -- you can prove to everyone that you are a success, even if you're still unhappy and growing older by the day.

Or maybe you just believe in supporting higher education. Either way.

The alumni who donate do so because they believe they got far more than their money's worth and they have enough money today to "pay it forward", so to speak. Because the return on investment for education is something that takes a non-deterministic amount of time to realize (both monetarily and in personal reflection), it's not coincidence that most most alumni donors do so decades after graduation.