Why do I get so clingy?

If I hang out with someone too much, I get so obsessed and clingy towards that person, and as soon as we stop hanging out, life just isn't the same. Happened a year ago. Me and this guy became brothers. I was at his house all the time. His family was like my family. We had a falling out, and it broke my heart. Recently I had a different friend living at my place for three weeks. We pretty much became brothers, sharing a room and a bed, hanging out 24/7 every single day. Now he's not here and I'm finding it hard to enjoy myself. What is wrong with me? Why can't I just be happy?