Kent summed it up best: to discourage it for fraud reasons. That's why family members may rearrange a suicide scene to make appear as murder or accident, in hopes of getting the insurance money.
2 main reasons why it is commonly illegal:
1. Moral/ Ethical (often religious- based) reasons - essentially that it is wrong because...(insert reason here)... and therefore it should also be illegal.
2. Cultural reasons, mostly concerned with the running of/ living in a society, so for instance, as mentioned above - for practical reasons in the law/ avoid fraud, etc.
You guys might be interested to know:
- In some countries it is not illegal to commit suicide (I believe this is still the case in Japan- unless changed recently - Historically, suicide in Japan is seen as an honorable thing to do. Kind of like the ultimate sacrifice, and many Japanese still regard it in this manner.)
-I recently took out Life insurance. Despite suicide being illegal here in Australia, I was surprised to find out that life insurance policies here will pay out if I were to commit suicide - but there is a catch- this does not apply for the first 13 months of the policy- a cooling off period to avoid the obvious!
I actually queried this, and I was told that suicide was treated like any other death, and that they were less concerned with the reason for the death, only with the fact that it had occured! (Not that I'm plannning on it

)