There is a good chance that your boyfriend's best friend gets mad at your boyfriend for all of the same things you get mad at him about. Best friends are almost like surrogate partners, without the sex, so the BF is most likely subjected to many of the same annoyances you are.
Light flirting, bonding, and spending time with the best friend of your significant other will enrich your relationship.
Competing with your boyfriend's best friend for his time and attention will only result in arguments and hurt feelings. It's best if you accept that he is an important