When we left Egypt, we went from being slaves to being free. But, we are now slaves again - to Hashem! So what's the difference? Chasam Sofer in the beginning of parshas Bechukosai says that the last of the berachos mentioned for following the Torah is 'voeshbor motos ulchem' - Hashem will break the chains of the yoke. But, he asks, this should be the FIRST of the berachos - not the last! First you break the chains, THEN and only then, can you have all the other brachos - rain, food, etc. He answers that Hashem is saying that He will cause us to NOT see the yoke of the Torah as a burden – but accept it as our reality, and this is His present for listening to the Torah. That's what it means "I will break the chains of your yoke."
So now we have the answer to a dilemma - how can we be slaves (to Hashem) and free at the same time? Which is it? Are we slaves or free or can we be both? The answer is yes, it's not a contradiction. Yes, we are slaves to Hashem in terms of having to listen to what He tells us to do. But this frees us up to be the real person we are supposed to be. By becoming slaves to Hashem's Will, we benefit ourselves. By being slaves to pharaoh, we benefit only pharaoh.
When I am able to have this outlook, I am the happiest. And when I am a slave to my desires, I am the saddest. Oh, it FEELS like I am happy - but it's only a dimyon - an artificial image of the true happiness. Who doesn't feel good to sin? "Ain adam chotay v’lo lo". But it's not the REAL happiness. The real happiness is when I am what Hashem created me for - and we can ONLY reach that stage when we become subservient to Hashem. That's when we become truly free, truly happy, and truly who we are.