First wheel is the person that is the lonely wheel in the front of a tricycle and the happy couple are represented as the back two wheels. Being the first wheel usually puts you in an uncomfortable position given then fact that you are forced to be around your friend and her/his mate. But remember, a tricycle can't function without the front wheel so the first wheel is a necessary component of the back two tires relationship.
Me: "Gosh Sammy, you are such the first wheel of this tricycle...look at Robby and Natalie that happy couple of tires back their and you're up here in the front such a lonesome tire...the first wheel"
Sammy: "Yea *sighs* it sucks being the first wheel...:("