Hello,
@Jaynee!
Monkeys are sacred to the Hindus. Every Tuesdays and Saturdays, Hindu tradition dictates that these monkeys be fed. They do so because the monkeys are the living representation of the Hindu God, Hanuman. From the latest data, there are about 50 million monkeys in India. Why did the monkeys become aggressive? Well, it's a mixture of human civilisation occupying into a once wild habitat and the tradition of humans feeding these monkeys.
As urbanisation expands, so does the living area of humans. The more we do that, the lesser space these monkeys have for their habitat. That's why they have moved into the city. This is evident in a city like Delhi, where more and more cases of monkey aggression are recorded every year. But other places like Uttar Pradesh and Himachal Pradesh are starting to have the same problems.
The stealing of food is the milder part really. There were cases of deaths after monkeys attacked individuals. Like the case of the Deputy Mayor who fell off his balcony after being attacked by monkeys. Or the 14-year old girl who fell off a building after being pursued by monkeys, she didn't die but she had serious injuries. It doesn't stop there, monkeys have been known to slap, bite and even get into people's homes and wreak havoc there.
The city of Delhi has tried various ways of solving the problem. They have tried trapping the monkeys which worked for a while. However, as monkeys are very smart, they did figure out how to avoid the traps. Next, they used Langur Monkeys to scare the smaller monkeys. But these types of monkeys are endangered so many were against the idea hence, it was also discontinued. They have also tried giving the monkeys their own food, which ended up costing more than they allot for the humans.
All of these solutions didn't work. The more that people feed the monkeys, the more dependent they become on us. I've read one article stating that these monkeys captured from the city, they were transferred into sanctuaries. About 98% of them rely on humans to feed them.
What do you do though in case you come face to face with these monkeys? Don't make eye to eye contact. Look for signs of aggression like yawning or smiling. Once they do that, walk away immediately. If it does attack you, show him superiority. Throw away stones or any heavy objects. If you have some kind of weapon, like an umbrella, use that. But still walk away while doing so. Don't swat the monkey if it lands on you, he might bite, just walk away slowly and it might just run away.
I hope this helps!