Because he wants you

Present your used panties wrapped in a nice gift pack to your bro. Everything will be cleared after that!

May be he loves to wear them

Because he is interested in you sexually

Just watch some popeye cartoons

Because he likes the taste and smell of your p****

Because he wants to f*** you

Your brother will be sniffing your knickers while pleasuring himself and probably looking for your pubic hairs.
How old are you both ?