Don't be too polite

Have you ever wondered why grumpy old people are grumpy old people? 
It's quite simple really--each person is only born with a certain amount of manors.  If we use too many of them when we're young, they'll all be gone by the time we're old.  It would be best to spread them out over time.  Maybe hold the door here--provide an unprompted thank you there.  It's harmless if spread out.  But, don't make if a habit or you'll wake up one day and that pleasant disposition may turn into an air of discontent.