Smells

There are many smells in our world but what makes certain smells smell bad or smell good? I think we can all agree that flowers, a BBQ, and the smell of a new car all smell good to us. But other smells such as garbage, BO, old shoes, and rotted food all smell bad. But what makes these things smell bad or good? Does a BBQ smell good because you know that burgers and hot dogs are being cooked or does it just smell good. Does garbage smell bad because you know there is a bunch of nasty stuff is in there? Is it all in our minds that make certain items smell good or do these things just smell good?