Is it human nature to be with the same person for so long?

Really, this is quite the rhetorical question, but I do ask myself this. I have several friends who have been in the same "type of situation" as me. The boring hum drum of daily life. The pressures of kids and maintaining relationships. "Life comes at you quick!" Many of my friends, as I am are real optimists. We strive to achieve and live life to its fullest. Getting lost in your childs stories or simply drowning in their smile. Making a game out of running late for the school's activities because one last customer had their own story to tell you! :-) These days that we live change us as individuals, from the inside out. We are not the person we were 10 years ago when we fell head over heels for that sexy man we're now married to! I've always wondered how much truth lies in, "I love you, but I'm not in love with you." I think I'm living that phrase. I love my husband dearly, I care for him deeply. I wish only the best for him, but he is not my livlihood. Your insight is truly appreciated.