There could be a few things going on here:
The first might be the natural flow of a long term relationship. Whether you are 16, 21, 30 or 55, most relationships (if not all!) do feel stale from time to time. It basically sucks when you realise it - but, it doesn't mean that it has to be the end. Realising it is basically just a flag that says "ok! crossroads - which path to take?!"
It could be your age. I'm 28 and I can honestly say that I feel very different to who I was at 21. My wants, desires, needs, goals have completely changed. When I was 21, I was in a long distance, long term relationship with a man I loved dearly. We were together for 5 years. We also opened our relationship up and I felt that I would be happier with a girl I'd fallen for. Cue that relationship - another three years! Then, when I was 26, all the exploration paid off and I met the one I currently think I'm right for! When I'm 35... i'll either still feel that way, or I'll have learnt more and be with an even better-suited partner!
The other possibility is plain old NRE - which feels anything but plain and anything but old. New people are exciting, because we are still learning about them, because they make us feel different, make us think about ourselves in new ways. NRE is chemical. Once the chemicals wear off, you are either left with something fantastic, or left wondering why on Earth you were so into that person.
You asked for personal experiences in terms of going through NRE and finding your primary partner a little more boring. I can give you a couple of examples, if it helps you.
When I was still with the ex boyfriend I told you about and we became open, so that I could also date the girl I was attracted to, I experienced what you are experiencing. She was fun, very sexual, lighthearted, talked about everything - everything I thought I wanted. A breath of fresh air. I loved my boyfriend with all of my soul - but my body and mind had been drifting over time. I would go for a drink with him and we would have very little to say to each other. I'd find myself wishing I was with her and felt awful about it. I broke up with him five years ago, broke up with her three years later... and looking back on the two relationships, I can honestly say that I made the right decision. I still love my ex boyfriend dearly - he is my best friend. But we are not compatible as romantic partners. I am still fond of my ex girlfriend, but we did not have enough of a connection after the NRE wore off, for me to consider her a partner for life.
With my current girlfriend, things feel different. We have very similar life goals. I admire her, desire her, love her. I am proud to be with her. I find our relationship incredibly fulfilling. She teaches me something new about the world every day, without even trying. It's how I felt for my ex boyfriend - only with all the elements: sexual, spiritual, emotional, practical. When I have been with other partners, even ones I have been very much into, the feeling of love for my GF actually increases. I have a sense of 'swelling' with love for her; which she says that she also feels. It just feels right, basically.
However, this doesn't mean I am never bored. Like you and your BF, me and my GF are long distance - I'm in the UK and she's in America. We spend three months together, then three months apart. We used to play Second Life together and so, we would always have something to chat about, always have gossip to share, always be able to share something together in general and not have to worry about long, boring silences, because we could sit on the phone, typing to other people, in silence, whilst still enjoying the closeness and the presence of each other. We stopped doing that recently and started to find that being on Skype wasn't so easy without something else to do together. We are going to start playing the game together again, because we are recognising that our boredom is coming from not sharing something together - immensely important in LDRs.
I cannot tell you what your feelings for your boyfriend come down to. It could be a compatibility issue. It could be that your brain will tell you this, because it needs a way out. Your friend, like the girl I fell for, might not actually be right for you - but might be the catalyst you need in order to explore things that you don't realise you need to explore yet.
My advice would be to cut back on the time with your friend. You are still in a relationship with your boyfriend and you do owe it to him to either give your relationship a fair shot, or think about bowing out gracefully. I am personally in favour of taking breaks in relationships, when emotions are confused. I took a break with my ex girlfriend, which became a break-up within two weeks, because it actually transpired that neither of us truly wanted to be together any more. Being on a break, not speaking for two weeks, made us realise that we didn't actually miss each other as much as we thought we might do... and that it was time to move on.
Me: (30f) open poly
GF: (40f) My long-term, long-distance partner
Hubby (37m): GF's husband
Garcon (26m): GF's submissive/third partner
“Peace comes from within. Do not seek it without." ~ Buddha