I've found that sometimes I want to write a compound sentence without a comma when it does not sound correct. This is an example:
Aden knew he was being greedy but was sure that he deserved an extra biscuit.
Aden knew he was being greedy, but he was sure that he deserved an extra biscuit.
Can someone tell me which of these two sentences is preferable and why? I'm not completely sure, and I hope that I'm not overlooking some obvious rules.