It's a common question and even we ask it when buying a home. Sometimes sellers are perfectly fine with relaying the answer to the buyers. Maybe it's just too small or maybe they're downsizing. The important thing is that the listing Realtor should not be relaying any information that puts the seller in a worse position.

For example, if the reason for selling is a divorce or job transfer, it might make the buyer think they can offer less because they want to "unload" the house. For this reason, don't be surprised to get some sort of a generic answer if you are a buyer asking the question.

If they are selling for a reason that can be detrimental to the house and/or it's value they have to disclose it. If they are selling because of the interstate they are going to put in behind the house they need to disclose that (at least in this state they do). If they are selling because the air conditioning quit working and they don't want to endure another summer they need to disclose that they know the air conditioning isn't working.

Look at it this way, how would you feel if you made an offer and the seller asked you why you were buying? If the answer was something to the effect of your lease is up in a month, you wouldn't want to provide that information because they might see it as a sign you don't have much time to keep looking for another home. The result could be a higher counter-offer.

It's normal to be curious, in fact I would be surprised if a buyer said "I don't care or want to know why they are selling".