My three cents: (upgraded due to inflation)
1) Rabbi Miller, in numerous talks, stresses the need to always walk with a smile, we’re always in show, conveying the portrait of an eved HaShem who is happy in His duty. So even when when we’re not in the best of moods, we should try and paste whatever kind of smile we can afford. We’re on HaShem’s propaganda committee.
2) Chassidim put more of a Dagash (focus) on the service of HaShem, rather then the development of one’s character. So even though one brings the other, a Chassid’s measure is not by what he has turned himself into, rather his measuring stick is what and how he serves His Creator. How he becomes a better person will eventually come about, however, that is not his goal. So every deed one does for HaShem is a cause for happiness. Another Mitzvah, another hour of limud haTorah, another aveira avoided. What a zchus to be able to effect such Nachas by HaShem. So of course its a source of great happiness.
3) The performance of a Mitzvah with Simcha is itself a Mitzvah. Which is cause for more happiness. Which is then cause for more happiness. Which is then cause for more happiness. Which is then cause for more happiness. Which is then cause for more happiness. Which is then cause for more happiness…….
(which brings to mind a story told over of someone who was not able to sell his stock of whisky in time for Pesach. So in middle of Pesach he made a huge bonfire and had his whole stock burnt. Then he hired musicians to play – to add to the Simcha Shel Mitzva. Details are etchy.. I think he was Zoche to something great in return…)