Jim Kirk had a long history with romantic entanglements.
Not relationships, mind you. More like... encounters. Some were boring and staid and ultimately short; some were fiery and exciting, for a time at least, and made for excellent additions to his ever-growing reputation in his academy years.
No matter how they started, however, they all petered out and ended the same way: with a loss of interest, certainly on his part but sometimes on theirs' as well, and the decision to move on.
Some of his partners had accepted this with a shrug and an offer to look them up on his next pass through town, others with slaps and bitter, barbed words.
Jim's not sure what they were expecting. He's hardly the type to have his interest captured by one single woman, man, or humanoid. No one was so ever-changing, fascinating, and accepting as to be able to capture and maintain his interest, and put up with his ingrained habits, for long.
Until Bones. Life just didn't get boring and predictable with his cantankerous, generous, brilliant lover. Especially since they got to lead that life on the Enterprise, a ship well known for its many and varied adventures that required every member of its excellent crew to survive.
"That's what I like about you, Bones," Kirk said as he raised his glass. "You keep me on my toes."