[16b bottom] Gemarah: When King Dovid realized that he had sinned, he did Teshuva immediately with proper remorse and regret.
Chasam Sofer: When we repent before G-d on Yom Kippur (and whenever we say Viduy) why do we sing it (shouldn’t it be sad and serious)? Because, a Jew is ALWAYS happy to be doing a Mitzvah (even when saying he/she is sorry).
click E’s Lesson (based on R’ Tatz): Not all Mitzvos are “enjoyable” and some even involve great pain and sorrow. However, when we know we are on the right path working towards a goal, the ‘service’ is always pleasurable [like agonizing through a tough workout for a physical goal]. That is how even when sitting Shiva (Lo Aleinu) we are able to fulfill “Ivdu Es Hashem Besimcha” (serving G-d joyously). We should never get discouraged, always be focused on our reasons and purpose for our actions, and not lose sight of our goals.