Check out if there is a medical problem. Sometimes these can decrease one's sex drive. You should have your testosterone checked out. The lust and the excitement and hormones of not having one person plays a part. Many people like the newness of getting to know people and they like the variety, but ultimately it is lonely and very hard to juggle.
Many people like to have that one on one though.
The challenge is to love one person and keep that exciting and fresh. Oxytocin is the cuddle hormone. Men love when you cuddle them, snuggle them, and touch them every place but their genitals. Kiss the neck and whisper in their ear. It will increase the sensation in the man's genitals by touching all the other places on him.