Question about my friend?

I have a friend who ive known since i was 4 whitch is now 11 years that ive known him , he comes over every weekend and sometimes during the weeks we do everything together summer trips etc when he sleeps over we sleep in the same bed (no were not gay ) but he has this thing were he wont sleep in just boxers he always has to have some kind of pants on i dont understand why , nether of us are gay in ANY way. they sleep with just boxers in the army i just dont understand i sleep in mine?