Why do people celebrate anniversary?

Most of us do it without thinking much about it, be it a happy event or a sad one. Here are some hypothesis I can think of:

1. To prevent us from forgetting about something important. A counter point on this is that if it's really important and has left an irreversible mark on you, you will not forget it anyway. Likewise, the fact you've forgotten about it means it's really not that important.

2. To take advantage of anniversaries to get something material, like diamonds for ladies.

3. To enjoy the good memories. But you don't have to do it only that day, nobody is preventing you from enjoy them anytime. Plus there is a diminishing marginal utility from it: how many times do you get the same excitement as you did for the first a few times?

4. To impress other people. See, we've been together for 30 years...

5. To get a change of mode.