We have a mitzva to focus on everything our family and friends do for us, whether providing help, compliments, or encouragement. It is a mitzva to acknowledge them too. Thanking shows we are grateful and understand their goodwill is not to be taken for granted. Gratitude increases blessing, as it reinforces good deeds and positive speech, and encourages people to carry on in this vein. In contrast, an ingrate is arrogant and blocks the flow of blessing. Also, ingrates will never be happy because they will always feel people do not treat them well enough and that they deserve more.
There is also a mitzva to be grateful to strangers and graciously thank them for any help or kind gestures as well. This includes thanking people whose kindness is performed in the course of doing their jobs: cashiers, salespeople, clerks, and the like. It is right to value every person who works and helps others.
