'cause baby you know me to the very core
okay geez i apologize in advance if this gets VERY GAY but i'm all tendy over my boys. ok so here's the deal:
they are best friends, bros and really close. katsuki is like eijirou's inspiration and he actually told eijirou to not give up and that his quirk was SOMETHING. katsuki, in fact, is not a guy with big words, compliments or even giving a good advice. however, kid's going through character development and so does eijirou.
the fact is that eijirou relies on katsuki A LOT. ever since katsuki got kidnapped, he tried EVERYTHING to get him back. katsuki was his close friend, someone he looked up at and could rely on. he calls everything katsuki does manly because he really thinks that katsuki is manly. okay now listen to me: this trope is overused and the fandom usually takes this as a reason to ship those two. katsuki is manly + eijirou likes many stuff = good ship/kiribaku.
don't get me wrong, it's what makes them a ship but they don't really go for the whole development about their relationship they have as friends first.
i have mentioned on my shipper that eijirou is the pining type and i can imagine him pining all over katsuki. he's that kinda guy and he just gushes all over him as well.
to be honest i really think they have a good dynamic and i really can see them as a main ship for eijirou.
i hope i could impress you guys!