Aziraphale was fully aware of how Crowley felt about him. After 6,000 years, you get to know a person very well - their emotions become obvious, no matter how hard they try to hide it. Well, that, and also Crowley wasn't exactly subtle. The long meaningful glances, the playful banter, the nickname. Aziraphale as fully aware of how the demon felt about him. He was also painfully aware of how he didn't feel the same. He loved Crowley, but it wasn't in the same way. He didn't feel romantic attraction towards him - or anyone, for that matter. Aziraphale did everything he could to return the feelings, but he just... couldn't. He loved Crowley, but not in the same way. And he couldn't shake the guilt he was feeling because of it.