Castiel had never understood human feelings, despite wanting to spend as much time as he could with them. He appreciated the company of humans more than that of his own celestial siblings. However, he never came around their way of functioning. All that crap about relationships, marriages with God as their only witness (if only they knew how irrelevant those marriages were in the eyes of God, who had better to do than to give his blessings to some love struck sheep) and even having children: Castiel revered his father's creations, but some of their attributes were simply beyond his understanding.
However, everything changed the day he grabbed his personal human Dean Winchester tight and freed him from purgatory. Everything changed the day he put his human's soul back together, piece after piece, seeing every aspect and every nuance of Dean. He saw his deepest secrets, his most hidden feelings, his fears, the happy moments he recalled… he knew Dean probably better than Dean knew himself. And Castiel now understood the meaning of the feeling 'love'.
He had fallen in love with this fragile little human, who tried to put up a strong façade, but was as vulnerable as a puppy inside. His stubbornness was his strongest attribute, enabling him to keep on fighting even when he felt like giving up, his love for his family was his most admirable virtue. Dean would do anything for his family, or rather the only family he had left, Sam and Bobby. Castiel smiled: he, too, would do anything to protect his family, only in Heaven matters were more…complicated so to say.
Castiel had learned that love had two sides: he had learned that it could overwhelm your heart to such an extent that you wished the moments shared with the loved ones would last forever. Love means protecting, comforting, teasing, understanding… but love also means suffering, being afraid, hesitating, making the wrong decisions. And all this because of that one person we believed we couldn't live without. Castiel had experienced each and every one of these feelings linked with his love for Dean. But he had experienced something that conflicted him in ways he never thought possible.
In fact, Castiel had given up on Dean so that his human could be happy.
He had witnessed his little human live a life really close to the 'apple pie life' Castiel had wished for Dean since the first time he had laid eyes on his soul. Because Dean deserved to be happy, to live a life free of monsters, free of hunting, free of suffering. He had eventually come to terms with his brother's death. He had got over Castiel. And he was happy with Lisa and Ben. He was close to being happy.
And this is the reason why Castiel decided not to disturb his little human's life by asking for his help. This is why Castiel didn't want to give into his selfish urge to appear to Dean and thus tearing him away from his newly found family. Castiel's heart hurt at the thought of never seeing Dean again, but he knew that in order for his human to be happy, this was the price he had to pay.
Castiel still had difficulties understanding each and every nuances of the human feelings, but he had started to accept them as being part of their every day lives. Turning his back on Dean working in the garden, Castiel faced the demon Crowley. He didn't turn back to look at his human one last time. He didn't want his last memory of Dean to be that of a greyish sad face and of dull green eyes that had seen too many horrors already.
Instead, Castiel brought back the memory of their first kiss to his mind before flying off after Crowley, heart heavy with the burden of the deal he was about to conclude with the demon.