As naive I can be, I always assumed people started inventing flying machines in order to "be like the birds".

In an earth-like world without any flying being (animal or plant), what could be the drive for people to glimpse the idea of flying?

Assume this is about a "Leonardo da Vinci" inventor, who is planning to create a machine to "go into the sky". What kind of simple observations and thoughts would be sufficient to at least spark this idea? Provided that gravity teaches that "what flies goes back on earth in no time", is it enough?