What I like most about it is making understand the nuances of the language, for example, when two words appear as synonyms, yet one would be used more in a different register or in a different situation than the other.
I used to live in France as a student several years back, and in order to meet my expenses, I started teaching Hindi. I did so in schools with little children as well as with adults who were interested in learning the language.
Bollywood! Another reason is when you have some part of your family coming from India, for example, a parent or spouse.
To get a grasp of it and start being conversational in it is actually quite easy. Gaining a mastery of it is, on the other hand, very difficult even for seasoned non-natives.
Many ask questions about Indian caste system, religions, spirituality. Some are interested in Hindi films, also known as Bollywood. Some are interested in Indian cultural exports to the world, like yoga or Indian cuisine.
The most difficult part is to make the student think in Hindi, rather than think in their language and then translate it into Hindi, which only results in a poor calque.
Someone very curious and eager to learn and try out something new. That would be valid for a student of any subject, I think, not just Hindi.
The grammar, especially remembering what is masculine gender and what is assigned feminine gender.
Watch a lot of Hindi films (or TV series) or listen to Hindi songs!
That depends on the student. Some become fluent within even half a year.