I am very late in this conversation Sorry.
Angel was thinking of Buffy's future when he left her not his soul though that does tie into her future with him.
Remember when Joyce paid him a visit?
"I don't have to tell you that you and Buffy are from different worlds."
"No you don't."
"She's had to deal with a lot. Grow up fast. Sometimes even I forget that she's still just a girl."
"And I'm old enough to be her ancestor."
"She's just starting out in life."
"I know. I think about it more now that she's staying in Sunnydale."
"Good, because when it comes to you Angel, she just like any other young woman in love. You're all she can see of tomorrow but I think we both know that there are some hard choices ahead. If she can't make them, you're going to have to. I know you care about her, I just hope you care enough."
This conversation is key because by hitting him with these difficult truths it's a prelude to his reasoning, the unbeknownst to us concerns that had been on his mind. He's always thought of Buffy first.
"I've been thinking about our future, and the more I do the more I feel like "us", you and I together...is unfair to you."
"You deserve more, you deserve something outside of demons and darkness."
"You should be with someone who can take you into the light, someone who can make love to you."
There you have it. As difficult as it was to give her up how much more difficult must it have been knowing that he could touch the woman he loves and make her feel good without becoming a monster?