Mm well the 'big deal' is not that he knows how to fight, but that he never told anyone, and that would make his friends feel a little betrayed, like he didn't trust them enough and kept things from them. The only one who thinks that being able to fight is a big deal is Love himself, because of his past (he desperately wants to 'belong', but since he could never belong anywhere BECAUSE of his fighting, this time he wanted to start a new life and didn't tell them, while still feeling guilty about it). Hope this was clear enough and not even more confusing for you >_<
He wanted to belong somewhere and he wanted to do so by not telling them that he could fight. Since he hid that fact though, he didn't want to tell them the truth because he felt like he was betraying them by not telling it in the first place. He also can't tell anyone what happened in the past because he just doesn't want to explain himself and let people know what happened and make them understand, make them emapthize.
Sorry for the dumb question but... Why do they make such a big deal over Love knowing how to fight..? Like alright he can fight but it’s not like he’s harming anyone now