He has a girlfriend so why is he kissing me?

I have this really good guy friend and since we had the same classes we hung out a lot. We're both really tactile people and people have told me that they think he likes me but since he has a girlfriend I thought it was impossible that that could be true. But the other week I got drunk and when I turned to kiss him on the cheek goodbye (this he does to me regularly but this was the first time I tried it) he turned his head and french kissed me! I brushed it off as a drunken mistake and pretended it didn't happen. I saw him a couple days later with friends and it seemed like nothing had changed. But when he drove me home he held my hand and even held my head and did an Eskimo kiss. Then when he dropped me off he leaned in to kiss me. I pushed him away and said he was feeling the alcohol he had at dinner. He then looked at me seriously and said that he was sober and he also was the other night. But if so why did he do it? He has a girlfriend! Since then even though I try to tell him what we were doing was wrong I've been swept up by his pace and kissed him several times. I know all he wants is to hook-up but then he does things that make me think twice like showing up at my place after he's worked all day because I had mentioned that I woke up with swollen eyes. What does he want from me?! He's been telling me he's taking a 5 week trip and asking if I'll miss him when he's gone. He texted me right up till his plane left. I thought he was going alone because that's what he led me to believe but now I find out he's gone with his girlfriend. If he didn't have a girlfriend It would seem like he wants to form a serious relationship with me but the fact is he already has a girlfriend. I've made my decision and I'm not continuing 'this' w.e it is. But I find myself wondering why he was doing this and the question keeps on nagging at me. Why does he call me when he's bored, when its late at night to see if I'm sleeping, tell me when he's going out with friends, picking up his mom, doing errands, it makes me feel like we're dating and I sometimes forget he has a girlfriend. The surprised looks people give us when they find that we're not dating and he has a girlfriend proves that I'm not just imagining it. I know he has no intention of leaving his girlfriend so why is he bothering with me? I'm so confused.
He has a girlfriend so why is he kissing me?
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