I don't understand how come people seem to hate the ending saying it was rushed. It was pretty much clear Hye Jin wasn't going to end up with Heedo. (Whose daughter wouldn't be able to recognize her own father while seeing his photos from past).
Also the seeds of growing differences could be seen from the start. Hye Jin don't have a graduation degree and he must give his 100% in his job to provide for himself and his family while Heedo already has PTSD from neglectance from his mother choosing her career over her family. Their decision to split was justified. You can see their relationship in contrast with Yu rim and Moon ji yung who were in worst situation than them (literally miles apart for years) but were still able to pull through. Their problems weren't in the way of their compatibility with each other.
Again another complain about people is why wasn't the dad shown. But this story is not about Hee do's husband. This story is about Ye jin and Heedo when they were 25 and 21.
Personally for me the ending is the best thing about the whole show that sets it apart for every other k drama. I was hoping for a 'How I met your mother' kind of ending, that Ye jin and Heedo would end up together after they got old. But again this story is about a distant summer they shared together. You are not the same person you were 20 years ago.