Many factors control such relationships, as they do with the way a business conducts with a customer. Many executives would agree on the importance of 'understanding' a customer, because the more you know about a person, the better you could make a sale ... or is it that simple?
Here's a simple analogy: I know who the US president is. I can recite his full family history, his achievements, with the help of the internet; but I don't get calls from him. Such is the simple differentiation between 'knowing' a customer and actually having an interactive 'relationship'.
Interestingly enough, before you have a social or business connection, you will need to have some knowledge about the person first. In return, the other party needs to know something about you as well, before anything substantial can be established. You don't go up to a lady and ask for her hand of marriage without actually knowing anything about her (you can do so, but it never lasts). The more knowledge you can share, the better the match fits. It's perfectly fine (within legal limits) to understand a customer more, about his needs and thoughts. But how much are you willing to give in return, as any healthy relationship demands? Certain companies can go to the extreme in terms of scraping every little detail you have.
I never like roadside surveys; they keep grilling me with detailed questions and I don't even know who they are or what they can offer in return.
Relationships are about exchange. Sophisticated marketers these days are more aware that constant one-way communications will ruin any goodwill faster than a lousy boyfriend. There are more innovative channels for the other party to voice what they think or need, in a sincere and unobtrusive way. No one likes pushy people, especially when they keep nagging constantly.
Good companies make sure any information collected is provided by the customer willingly. The customer believes he will be treated with respect and trust.
Relationships are developed and maintained mutually. We may have a good understanding of each other, but that doesn't mean I have to be in love with you.
Relationships are about matching. It's better to let go early than to endure painful consequences after an unfruitful outcome. The last thing a company should do is to make enemies with potential allies. If the customer asks you to stop emailing, do so.
And what happens when you already have a relationship? You go back to step one and continue the cycle. Oh, and this works for your social life as well.