Source: naanaach.blogspot.com A Great Mitzvah to Always Be in Simchah
"Upon the asor [ten-stringed instrument]… for You have made me simchah [joyful] with Your deeds…" (Psalms 92:4-5). It is a great mitzvah to always be in simchah and to strengthen oneself with all one's might to remove atzvus and marah shechorah...