Why are things like this between us?

I've known this guy ever since I was a little girl in elementary school. We were great friends in grade four, but we unfortunately went different ways throughout the rest of the year. (We still talked every now and then, but we weren't as close). We recently went on a school trip (for 2 weeks) and at first we didn't even speak to each other and avoided each other but after a few days we warmed up and we practically hung out the whole time and talked the whole 2 weeks. Now that the trip is over things are back to normal and we're uncomfortable and don't talk anymore. My point is, whenever we're in a class together, or are around each other for awhile we both warm up and talk and everything goes smoothy and we have a great time with each other. But when we see each other around school/etc we're really awkward and don't talk at all. Why are things like this ?