You notice that you feel really natural together, as if you had been friends forever when you just met. There is some deep level of commonality you share that is beneficial for both of you, so much so that you might end up excluding your other friends in favor of spending time just with each other. This is hard for your lover, especially, to resist, because you give them so much emotional support.
You just know what they're thinking and feeling lots of times, and you are able to voice their emotions better than they can themselves. You are a sort of filter for their emotions, and you really help them to process them better than they might on their own. You may do this for them, and they'll return the favor for you. You both feel accepted, nurtured and understood, and so continuing your relationship long-term is a natural process. You notice that you're often in the right place at the right time together -- a sense of good luck pervades this relationship. Even the way you first met probably seemed like that to you. The only thing to beware of here is that you are so in tandem with each other that you might start to take that bond for granted. Don't! It's a gift, one that should inspire gratitude.