I like it. The characters feel organic and not played up. Sure they might have elements of a stereotypical harem girls but in all reality there isnt some instant fall in love. Or some play it up for cheap laughs with a boob grab or something like that. They argue like adults and generally feel...
Even if it is just him being friendly. Friendship can often turn into romance. So the fact that his guard wasn't up at all means he is getting closer to her.
Cute. Wholesome. Love it. It moves slow but slice of life stories do. The characters do develop. But more about their relationship. People don't just change all the sudden. When you were in 11th grade. Did you magically turn into a different person in the time period of 1 year? Not really. But...