Why am I developing feelings for my fiance's best friend?

I have been in a relationship for 10 years, and he is my one and only partner. I love my fiancé with all my life and do not plan on ever leaving, though I have stated liking his best friend. As mentioned, I would never cheat, or leave my partner as he is the best person I know of. There is no point selling him, as I know he is perfect, but my heart skips when I see his friend. I want to know why have I started developing these feelings for someone else, after so long?

He's been your only partner and going from your profile age, this means you've been with him since you were 15! It seems totally natural to me that at some stage you will find yourself attracted to others. Of course this doesn't mean that you're not interested in your fiance anymore, but seeing as you probably haven't ever vented or explored curiosities with other people, that side of you has been somewhat repressed. I wouldn't be surprised if your fiance has experienced similar feelings.

Part of being human is wanting to explore the unknown and believe it or not, this includes exploring other people. Don't be hard on yourself for being human!

Your fiancee gets the same feelings when he sees a hot babe in a bikini on a beach. Men are attracted to sight. Women are attracted to a man's presence. Neither one of one have to act on them. But remember, for the rest of your lives, attraction is not a choice. Acting on it, is.

Because you're human. Also, 10 years is a long time and it's even longer between a 15 yr old and a 25 yr old, you're a completely different person now to how you were.
It's normal to find other people attractive to but it sounds a bit like you're trying to convince yourself that he's perfect. Maybe you didn't have that time to explore a bit before you settled down and my personal experience is that these feelings don't go away.

Andrula, have you heard of polyamory? It is a controversial idea to some and goes against many of the social mores we have grown up to accept. But I think it has a lot of merit.

The big advantage I see to polyamory is that is much more natural. I find the notion of monogamy rather artificial—your current feelings are evidence of this. By contrast I find the idea of expecting an exclusive commitment rather selfish, perhaps even narcissistic. Monogamy connotes the belief that love is limited and so must attributed to one person else there won’t be enough. Monogamy runs against the expression love for self and individuality by failing to appreciate the value of our individual differences and the advantages of specialization therein. So instead of having one love who is really good with money and another who is really good with children you demand both those skills from one person. Is this not quite a burden for that one person? And also somewhat inefficient: trying to force a square peg into a round hole when there are plenty of round pegs around.

I should mention polyamory has nothing to do with promiscuity. You are not polyamorous if you have one partner and go round cheating on him all the time. You are polyamorous if you are faithful to more than one person and you honest and open with your existing partners when you have strong desires to add other people to the relationship.

Another advantages to polyamory: more parents for children; less loss associated with the death of one; children get a greater sense of perspective by learning from more than 2 parents. Of course there is only ever two biological parents for any single child but the word parenting doesn’t mean making babies, it means raising them.

I don't know a lot about polyamory, but our ancestors weren't monogamous. I'm more of the idea that most of us are naturally monogamous, but not with the same person for all of your adult life. I think we need different types of people for different times in our lives.