Nothing hard to understand here....
You said that you "replied with a reasonable counter offer"
1) What you consider reasonable, they may not
2) A lot of end-users come to the table to buy, not negotiate, and on a somewhat regular basis, open communication with what amounts to their best offer. If they get a "counter offer", they simply move along to another suitable name (it's much rarer than most "domainers" seem to think, that any given name is the be-all-end-all for a given business. There are usually 100 suitable alternatives and the first person to not get greedy and take their offer will be the one who rings the cash register)
There is always risk of ending the deal with counter-offering just for the sport of it, particularly if the initial offer was a fair one. It's rooted in greed... "Well, if they're willing to pay X, then obviously, they must be willing to pay Y!!!" which just isn't always the case.
If their initial offer wasn't reasonable and your counter-offer was, then there's nothing to worry about. They were only willing to pay a price that wasn't acceptable to you, so you have nothing to agonize over. Sometimes, even big businesses don't want to pay a lot for a domain...
However, if their initial offer WAS something you would've taken- had you known that a counter-offer would have resulted in breaking off communications- then you simply got the bad end of the risk associated with counter-offering against reasonable first offer. They didn't come to negotiate a price, they came to buy a domain name and when you informed them of the price you were willing to take (which is how they viewed your "counter offer" - a lot of people don't play negotiation games), they simply walked away.
The lesson to be learned here is this.
Imagine that the initial offer that's being made is the ONLY offer you will receive. Knowing that, is it an offer you would take? If yes, then you should strongly lean towards taking it. If no, then you have nothing to lose by counter-offering since you will not be selling the name for their offered price anyway.
Counter-offering against reasonable offers is a mistake we all have made, however, many of us have learned from our mistakes, as we continue to renew names we wish we would've sold to 'that guy who wanted to buy it back in 2003." Sometimes, the first offer you get is the only offer you'll ever get for a domain.
