Why does he flirt with me and then not look at me the next day?

So why do guys do this and what does it mean? Well the guy I like I think might like me back, he has done too many things for me to type here much less remember all of them. But the most recent one is yesterday he was standing with his friends and then when I walked off to go to the car to leave school he walked off right behind me, which he usually waits until all his friend leave before leaving. Well all his friends were calling his name so that makes me think he either left w/o saying anything or left earlier than normal. Well anyway when I got to the car I opened the back door to put my stuff in. There was a car, then my car with the door open, There really wasn't much room. Well the guy I liked was behind me (not like RIGHT behind me) and he came and went through the aisle where I was at. It was a tight squeeze so I had to move over to let him by. He could have gone around any car in the parking lot but he went where I was at, the tightest place. But anyway he did that and something else big yesterday but I'm not gonna type it. But today he didn't even look at me, he usually looks at me as we pass in the halls but not today.
I know it sounds like be probably doesn't like me based and that one story but there has been tons of other signs like calling me good looking and all his friends teased him about liking me.
But my question is:
Why would one day he do flirty stuff and show signs that he likes me and then the next day not even look at me? This doesn't happen often, because he usually looks at me everyday at the minimum, but some days he just doesn't look at me at all. Why does he do this?