You are emotionally in sync, having been able to tune in to each other's emotional rhythms from early on in the relationship. For this reason, even though you're very different on many levels, you are able to forge a strong, intimate bond because you just get each other on a deep and intuitive level. You share a simple affection for each other that fosters a real sense of togetherness.
You just really like each other, and you trust each other. You don't often have to wonder where you stand with each other, because you know. When your lover is upset, you understand why, even if they aren't able to talk about it. This could be more true of one person than the other, which is okay -- that just means that this partner will be the one who keeps you two together, united on an emotional plane. If the intimacy between you ever breaks down, this partner will be able to reach out, either physically or emotionally, and bridge the gap again, which is a very useful skill in a relationship during a fight or other estrangement. Even if there are other aspects between you that are more troublesome, this one helps to smooth them over, because it creates a feeling of understanding and sympathy between you.