I'm not sure if someone mentioned this above, but I can't think of any good reason to exercise a put option. Sometimes people exercise a call option in order to capture a dividend, but when you exercise a put option, you're throwing away all time premium (extrinsic value) left in the put option. Since american options can be exercised at any time, the premium of the put option will contain both intrinsic (if the option is ITM) and extrinsic (time premium). The intrinsic value will always be equal to the amount that the option is ITM because someone holding the option can exercise it at any time and always get that amount. Only case that I can think of to exercise is if the option is way deep in the money or not liquid so that the bid / ask spread is wide, but even then if the option is priced just one cent or two less than the intrinsic value, someone will buy as they could use it as part of a risk free trade. Also, most brokers aside from IB charge an extra fee for exercise / assignment.
Apparently the ONLY incentive to exercise a put option early is if the interest rate is extremely high and selling the stock via exercising the put option at the strike price and then investing the proceeds in interest-bearing investment would earn large enough return than all the loss in intrinsic and extrinsic values on the options and all the potential capital gains from holding the stock. http://tradingmarkets.com/recent/learn_when_you_should_exercise_an_option_early-640882.html
But since the interest rate is so low in North America there should be no incentive to exercise your put option aside from REALLY wanting to get rid of your stock but nevertheless I have had put options exercised against me on quite a few occasions when they went into money. The Options Clearing facility of both Canada and US have this policy that any options ITM for just $0.01 is AUTOMATICALLY exercised UNLESS the trader gave explicit instruction to the broker to never exercise any options. That's good that just allows me to make a killing when I am able to sell them at much higher profit later on as I did on all of the occasions.
