There are some that translate the pasuk "עִבְדוּ אֶת־יְהֹוָה בְּשִׂמְחָה" not as "serve Hashem with joy", but "if you serve Hashem, you will be b'simcha"
The truth behind this is profound: it is not possible to attain happiness through self-service. In fact, when we are concerned about our own needs, we are definitionally not happy, but rather we are anxious, stressed and depressed. After many years of taking this approach, and filling in the void of happiness with yeitzer haras like food and cheap thrills, we eventually start to question what is the point.
Why were we created? So that we should be given needs and problems to worry about? We meant to spend our whole life pursuing our needs, under huge effort for little reward? The younger people realise that this isn't the way, the better.
Happiness comes when we realise that we didn't create ourselves, or our needs. Someone else created us, and He asks us to do something for Him. What a relief! Now we know our purpose, which is the first pre-requisite to finding happiness! Secondly, we have won the lottery! We have an opportunity to serve Hashem! Whatever He needs, we will do, and that's where we will find true, lasting simcha.
Hopefully we can learn this before we discover it through life experience, such as having a spouse and children to serve. Many people discover the joy of service at that point in their lives, and often it is quite too late - a lot of damage done. Hatzlacha!
All of this is based on Rabbi Manis Friedman's lectures on this huge topic, so here is a playlist if you'd like to hear more. The sources are mainly chassidus, as he is a Chabad chassid.
