Can somebody explain me why he feels guilty please?

He might be falling in love and he met him was a 'trick', or He was never honest about things, or he feels like a conartist in this sense, or something of the like

Ah okay, thank you

Just read the earlier chapters first again... It will be explain his state of mind..

If you hate to read: Black hair set up a teaching lesson of teaching how to have sex... And then the brown hair guy, drunk at that time, responded that he wants to apply...

So, the black hair guy, not really revealing his real identity, introduced himself and responded to brown's inquiry, (that is why Black's friends call him a con artist) and black and brown met... And the teaching begins..
I think fooling a puppy boy made black regret trying to fool the brown haired guy

Lol thank you very much, your explanation was fantastic. And it’s not that I hate reading, it’s just that English is not my mother tongue so I was not sure if reading again the first chapter was a good idea since I didn’t understand the first time. Anyway thank you very much (⌒▽⌒)