Where did it start is not clear but most folks think Nashik is the origin. In older days, folks used to make "bhakri", a form of bread made of Jawar/Bajra and used to take misal, which used to provide a good source of protein as it was made of moong dal "usal". For taste, they started putting sev, mixture, onion, lemon etc to that. Some misal also has potato as source of carb.
Pav was not originally associated with misal. Misal in itself was almost a complete meal but folks wanted something with it, so it bhakri they made was not sufficient, then they started adding pav to it.
In Nashik, one can take unlimited tari with misal and so, the idea was to keep taking tari, and keep eating pav with that, thats where the "1 misal, 12 pav" came to being.
Another important consideration with misal is that it has to be "zanzanat", means after eating it, people should start perspiring because of spiciness. There is of course taste of the "usal" in there somewhere, but the typically misal is heavily spiked with mirch, spice. One can round off the taste with yogurt and lemon. That helps for a few not so bold ones.
Misal par has overtime become a staple food for folks in Maharashtra. Other than Nashik, misal pav is available and famous in Nagpur, Kolhapur, Mumbai/Thane and Pune also.
So, whether you are the bold one or not so bold one, go and eat misal-pav as it is a healthy, hearty food.