Why can't they be rational when rationality is presented?

I became so mad when someone had the shittiest argument and my school mate with whom I hang out with thought that it was a decent argument.

The argument had nothing but generalizations; no facts supporting it; and the reasons why the argument was was so bad that it made me in-deliberately touch my glabella.

When confronted I asked him to name one thing that didn't generalize (he told me there wasn't a single generalization), but he just laughed it off and whatever.

Just why? I reckon that at least some of you can tell me why.