You notice that you feel really natural together, as if you've been friends forever, even from the time you first 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 you to resist, because your lover gives you so much emotional support.
They just know what you're thinking and feeling lots of times, and they're able to voice your emotions better than you can yourself. They're a sort of filter for your emotions, and they really help you to process them better than you might on your own. They do this for you, and you return the favor for them. You both feel accepted, nurtured and understood, and so continuing your relationship long-term is a real possibility here. 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 seems like that to you. The only thing to beware of here is that you're 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.