Please do keep in mind the way the practice of promoting dive courses works. Yes, the dive store is interested in selling courses, every store sells something. The upside to having people continue on in their training is that it keeps them in the water, keeps them learning, and progressing as a diver.
You did mention that you wanted to prove to the store that you were not just a casual diver. They appear to have seen that, and are encouraging you to continue to do so. Is that supposed to be a bad thing? Remember, most new divers never dive again beyond 16 months after their OW certification date.
For most recreationally-focused shops having someone become a divemaster is the natural progression of courses after AOW. Some people are ready to START that training with less dives than others - each person should be assessed individually. No one gets to finish the DM course with less than 60 dives (for PADI, anyway) and for the average diver that is ALOT of diving, while for others it's half a season or less. I doubt what the owner is saying is that you are ready to BE a divemaster after 18 dives, but ready to LEARN to be a divemaster.
In their way, the dive store owner is trying to encourage you, it's what they do, hopefully to the benefit of both you and him. If it only benefits him then you will be a disgruntled customer and leave.
Just playing devil's advocate folks. We need to keep both sides of an issue in sight at all times.