Been wondering that myself. They most definitely love each other, no matter WHAT either of them says.
Diesel never said that, he was just wonder about this and was not sure about source of his feelings toward Ein.
...and Ein said, that s is not love, when he say about Minsuk and Yongjae relationship (last Minsuk boyfriend), not about this what is between him and Diesel.
http://www.mangago.zone/read-manga/wolf_in_the_house/nmt/c019/36/
On this page and the one after, Diesel is saying that Ein killed the person that was most important to Diesel. That makes it sound as though he was in love with that other person and not Ein.
Cause he was in in love in person, who Ein killed. Diesel also said, that his love for Ein grows with each passing day and he s not sure, that he is in really in love or it s just canine instincts. He wonder, since he fall for someone, who killed his past love, that means he would love anyone who takes care of him and with who would be? That his canine instinct force him to this? Diesel doesn t know, he is not sure, but he never said, that what is between him and Ein isn t love.
they're so deep in love how could he even say it's not love