It seems the priest has a misunderstanding of the reason it is law. In universal concurrence, yes, missing the ‘holy day’, mass, has certainly been treated as at least grossly disliked or as a mortal sin, even from the very foundations of the Church. It stems as a continuation of the Mosaic Laws in the proclamation of the 10 commandments: to keep the sabbath holy,. That itself is a law, which Jesus frequently preached that we should practice and keep. Indeed we go to mass to give thanks, as is the meaning of the word Eucharistia, to give thanks, and as HelenRose said, we do it as a response to the call from God, it is what he likes us to do in worship to him, so we should, indeed, follow that wish. We live to serve.
All the best, hope this helps,