No man has ever gotten a woman into bed by explaining why she should sleep with him. That's the red pill praxeology that we can all agree upon. It's not a hard rule for every situation though. You might be explaining to your roommate as a courtesy and he's just uncooperative or you might be making excuses, I dunno.

To a man: naah, I don't think I'll do it.

Him: why not?

You: it's not for me.

To a girl: no.

Her: why

You: look at her then continue with what you are doing

You've probably just read that you shouldnt explain yourself but you need to understand why, how and then internalize it. It takes practice. Read the sidebar material, then The Rational Male, and WISNIFG, and No More Mister Nice Guy. These are books so it's a process.

Stop caring about what your roommate thinks if he's not a good friend of yours. Just stop explaining yourself and say No. No explanation needed. And take care of your shit so he doesnt have things to whine about it if it's stuff you actually need to be doing.

My recommendation would be to practice with text messages. You have more time to generate an appropriate response. From there apply it in person.

"I understand why XYZ would make you feel that way".

This is a key basic statement. No explanation, no bitching, nothing but smoke.

'No' is a full sentence