It didn't fail, they are together. It's a yaoi what did you expect? Plus to love someone for 4 years after you break up is really something.
They had a huge misunderstanding and both feared love. Now it is four years later and they both grew up and realized what they really wanted and that is each other. For four years Junyoung slept with other guys but NEVER dated any of them. His true love is Shihoon. I believe trust and misunderstanding is what this what about and are finally reunited so they can do it right this time. But obviously these two can only love each other. I loved it.
They were always awkward around each other but now after 4 years it is even wore. And of course the first thing they do is sex thats why their relationship failed in the first place (〜￣△￣)〜