Two of Troy's tendencies play into his decision to cheat on Rose with Alberta.
First, Troy has a strong tendency to excuse himself. We see this tendency in his fixation on how he was wronged in his baseball career.
The fifteen years that Troy spent in prison made him too old for the major leagues. Troy ignores this argument, since to acknowledge that he was too old is to accept partial responsibility for not being able to play...
For Troy, blaming others allows him to cast himself as innocent in his own mind. He says as much when explaining to Rose his reasons for cheating with Alberta.
The marriage is an unending series of obligations for him, with no possibility for laughter and ease. Though he does not blame Rose for this, he also refuses to apologize for cheating. He says his conscience is clear.
The pressures of his life as a provider have led him, quite naturally in his view, to find an outlet, as he tells his wife.
His response to her anger and pain is an admission that the other woman offers an escape from his responsibilities. She makes him forget the endless repetition of his life for a few moments.
In going outside of the marriage, Troy is enacting a second tendency in his character - to exceed limits and fight against boundaries. Troy's biography is marked by his efforts (successful and not) to break down barriers.
His bitterness regarding baseball stems from his failure to break the color barrier and play major league baseball. His baseball career in itself was characterized by his home runs - a symbolic victory over the barriers set up at the edges of the game.
For these reasons, Troy is drawn to go outside of his marriage and able to excuse himself from guilt as well.
Because he said this gave him release from stress,but just caused him even more.