We don’t HAVE to do lots of things. We don’t HAVE to brush our teeth. We don’t HAVE to do things that have been medically proven to extend and improve the quality of our life. However, we DO have to breathe, eat, and drink. Without the minimum effort, life will cease. However, if we only eat junk food, breathe only polluted air, and drink tainted water, our lives will neither be abundant, nor particularly pleasurable.
So it is with our immortal lives. We can meet the bare requirements of Eucharist and Confession once a year, and put your butt in the pew every Sunday because we’re told to. But what is that telling God? That He’s worth only the minimum? And yes, if you’re only showing up to weekly Mass because “you have to”, then, operating under that belief, why bother? Why bother being religious at all? After all, God gave us Free Will, and has clearly shown how much He values it. So why violate it by performing religious duties only because you HAVE to?
You say that you understand that Mass is an important part of the Church, but then fail to see why it would be a big deal to miss it when “things come up”. The Church Herself recognizes there are times when one will miss Mass- weather, illness, etc., and under those circumstances, it’s NOT a mortal sin to miss. However, how many times do those circumstances present themselves? What sort of “things” coming up are you talking about? Sleeping in late? Scheduling too many errands, outings, social events on Sunday?
The whole question centers around what is the main priority in your life. If you view reception of the Eucharist as something you HAVE to or DON’T have to do, if there seems to be a “big fuss” about Mass, or if things keep coming up to prevent you from going to Sunday Mass, than God is clearly not the main priority in your life. And as both the Old Testament, the Ten Commandments, and Our Lord’s own words bear out, that is clearly a grave trespass, bringing with it mortal sin.