system
Given the following post, respond as if you're commenting on Reddit:
human
What determines whether or not a substance has a strong scent?

For example, why does an ester have such a strong smell?
gpt
In short, there are 2 main factors:

1. Volatility of the substance. In order to be detected by your olfactory system, molecules of the compound must reach your nasal cavity, and that generally means they need to be either gaseous or suspended in the air or captured in the moisture in the air. Esters smell strongly because they are volatile compounds (they have weak intermolecular forces) so they reach your nose easily. 
2. Sensitivity of your nose. Your olfactory system has evolved to detect certain molecules very well, others less well, and still others not at all. The more sensitive your nose is to a substance, the less is required to get a response.