איזהו חסיד, המתחסד עם קונו

This teaching from the Baal Shem Tov appears to encapsulate Chassidus in just two sentences:

Whatever you do, have in mind to give gratification to your Creator, blessed be He, and do not think - even a little - of your own needs. Even the expectation of personal delight from your avodas Hashem is an ulterior motive for one’s own concerns.

(Tzava'as Harivash #11)