To some people "sorry" may seem to be the hardest word. But to me "thank you" seems to be the hardest word for so many people. In Israel I met so many people in whose daily lexicon this expression didn't exist. Unfortunately, even in the country where I live now, I meet so many such people, who are otherwise well mannered. In both countries the majority of the people I've interacted with have just kept silent in those contexts where I would never fail to say "thank you."
I never complain to these people as I never take this personally. Nor do I even point this out to them as they seem unaware of a deep, spiritual, consequence of this verbal action of theirs, or to be more precise, lack thereof, for which I'm truly sorry. Actually, I've even stopped expecting this hardest from others in order to maintain my peace of mind.
Many teachings of spirituality, including Chassidus, emphasize the importance of not only being grateful for any favor one receives, however small it may be, but also verbally expressing that gratitude. This must be the reason, or one of the main reasons, why the Rebbe said that his most favorite prayer is מודה אני. Since I heard this, I've been reciting this prayer first thing every morning to verbally express my gratitude for having my soul restored to my body.
What is the deep, spiritual, implication of verbally expressing gratitude constantly? My understanding is that this prepares a "container" for receiving blessings that fill the universe by making the flow of spiritual energy smooth and preventing spiritual mud from filling this container as well as the channel that leads to it.