Dean always knew how to hide his feelings.
Lies, regrets, you never saw anything past his green eyes.
He knew he had to stay strong for Sam;
He knew that if all the trouble with being Lucifer's vessel became too much for his little brother to handle, he would break down.
And then who would take care of him?
Every night, drunk with sweet kisses and his brother's scent, Dean burries himself in his pleasure, making himself forget about how wrong the whole situation is, and how guilty he feels for finally giving in.
After all, if he keeps pretending he doesn't love Sam much more than he should, it will became true, sooner or later.
As much as he claims to be happy with the ladies, staying till late hours in clubs and drinking strong alcohol, deep in his glassy eyes, Sam's face remains.