6 is an arbitrarily chosen number, though. If you want to be really secure you can wait for more, but realistically, you are safe with fewer than 6. Many exchanges wait for 6, but some will be happy with 3. You can play around with the numbers on this site: https://people.xiph.org/~greg/attack_success.html. If an attacker has 10% of the hashrate (proportion of 0.1), and you wait for 6 confirmations, then they only have a 0.02% chance of reversing a transaction.
Personally, it depends on the amount of bitcoin you are receiving. If I was sending or receiving a couple of bucks, I'd be happy after one or two confirmations, because if the transaction was reversed it's not a big deal. If, on the other hand, I was sending or receiving a few thousands dollars worth, then I'd be waiting for 6 confirmations. It's entirely dependent on what you think is acceptable risk.