Yeah, it was the most popular Naruto shipp for awhile in Japan (and is still on the top 10 pairings, I think!). And the shipping "business" it's mostly a girl thing so idk what to say about that.
Uhh, what do I like about them? So many things desu. Their personalities fit really well together, in my opinion. They're so different and yet they have so many things in common at the same time. Iruka's all heart, emotions and honesty and Kakashi is all mind and he helds his secrets and emotions very tightly. Kakashi helped Iruka with his teacher career and it's very supportive with him. I think Iruka is someone who would be able to help him open up and heal. I prob have +100 reasons but my english is terrible and I don't want to bore you haha.
And for where does it come from... There was this little moment in season 1 after their "fight" over the Chuunin Exams. At some point Kakashi compliments Iruka and they both blush. It was super cute for me (and I guess it was like that for a lot of people). They didn't interact much in the series after that but the little moments they shared were nice (Kakashi saving Iruka from Pain, those flashbacks/fillers with Kakashi saving Iruka again and helping him with Naruto). The wedding chapters were also extremely adorable (it looked like they were the ones getting married, and not Naruto and Hinata lol). There's also a chapter in Boruto when Iruka yells at Kakashi for being too mean to his students and it's both cute and hilarious!
This post explains it better tho: https://mightyb013.tumblr.com/post/168486749311/look-at-kakashi-and-irukas-relationship-they-meet
Yikes! This turned out longer that I planned, sorry!