DH and I have now been together over a decade and have 4 dc. We have been through some extraordinary ups and downs but have come through these adventures stronger, closer and more in love. I know him better and love him more each day and as he does me. WWIFN, puts it very well: [couples] "learn" how to be eachother's soul mate. I think DH and I are like that.
When we first met there was a truck load of lustiness and a lot of 'I like yous' but that has now matured to a deeper love borne of shared experiences, mutual respect and common goals. The lust is still there, but it is better now because I am much more open about what I want and the things we do now I was too nervous/shy to discuss in the early days.
Do I believe that DH is the only one on this earth for me? No. I imagine if, God forbid, he died I would be able to move on as long as there was love, mutual respect, loyalty and good sex. However, my DH is unique because with him I have a lot of my 'firsts', I will never have that with anyone else. He is also a very nice human being, a lovely, lovely man. He is honourable, dignified and graceful and he loves me very much.
Yes, I like what WWIFN said. I think we have learned to be each others soulmates. If we continue to avoid being complacent, if we continue to love, like and respect each other then I think we will remain 'soulmates'.
I may not believe in the OP's version of soulmates but I think it is very lovely that she and some of the other posters have such a romantic view of relationships.