It's a shame it's ending, but you kind of can only put that much story into those 3 years. They either need to move forward into after-school years, or end it. So they did. As other people saying, it was so good because it was romance\slice of life without the usual BS plot stuff romance usually have. It's especially obvious with Horimiya anime, where they basically make fun of usual romantic plot crap. Too bad there is nothing similar to read.