Dean Winchester has faced many things relationships are one of them. AJ is like Deans brother so dean leaves AJ with a huge task of finding an demon that killed Sam. AJ faces his own choices of his fear.
Imagine meeting Dean Winchester back in highschool and things get so complicated you have to part ways with him, only to meet him again later in your life.
"If you love someone, you should always go after them" people often say but I think if you are lucky enough for fate to care about you, life will always find a way to put you back together.