I've noticed that when we have somebody to depend on, our emotions are multiplied. It's like every thing we do has two dimensions to it, two people doubling the amount of happiness that only one could dream of. We laugh together, dream together, smile together; yet we cry together, worry together, frown together. It's almost like, when you're in pain, half of that emotion is shared between you and them, and you don't have to face your troubles completely alone. A hug is like a force of intense trust that for a second only belongs to the tight force that you've created. And it's good, for the most part, that we are able to come together in a way that lessens pain and doubles happiness, but there's also a downside.
When that significant other no longer feels your force, you are alone. And now you have to hold the entirety of your hurt, and are only able to experience happiness as to what your heart can contain. And that causes people to build up walls so that they don't have to be hurt and experience said hurt, which also degrades the amount of happiness that they feel. So sometimes, yes, the saddest people often have the happiest memories.