Many people believe in the concept of a "soulmate". Do I need to share
I'm wary of these sorts of comparatives: best, closest.
We all have many relationships that mean a great deal to us, and we do not need to make sure that one of them is "best", "closest". Indeed, if there is anything I've learned about relationships, it is how destructive those sorts of expectations can be. One certainly should not feel that if some interest one has isn't shared by one's partner, then it isn't worthwhile, or has to be sacrificed to the relationship, or what have you. That kind of thing starts to sound to me kind of clingy and possessive.
There may be some counterexamples, but in most healthy couples I know, both partners have interests that are not shared (or not really embraced) by the other partner, and their freedom to explore and nurture those interests with other friends or family is part of what keeps them growing, both individually and as a couple.