A lot of reasons. Some include (but obviously are not limited to):

1. To distract from severe emotional pain

2. “Testing the waters” for suicide

3. Out of curiosity

4. To fit in with others

5. For attention

I myself have never done this, but I used to know people that did. Typically people who don’t do it for attention try to hide it and generally do a pretty good job at keeping it hidden. People that make it obvious generally are doing it for attention, or they are at such a desperate time in their life that they are screaming for help.

Mental issues are incredibly varied and widely misunderstood. Every human brain is different and thats why it is very difficult for even health experts to explain why they happen, or even to come up with an exhaustive, definitive list of reasons.