Sannah is right that the reason it is easier is that it releases endorphins. Most medications do that as well but take longer.

If you ever decide that SI isn't the way you want to manage I would suggest looking up a bit about endorphins and how you can release them. As puzzclar said exercise is a good one. For some people they can also have similar 'release' through activities such as art, writing etc.

But I definitely do understand. SI was something I relied on for a long time to manage my low moods.