Why are the Immaculate Conception and the Assumption of Mary such a big deal that all Catholics must believe in them?

As described in [this answer](https://christianity.stackexchange.com/a/2952/16688) to a question about infallible papal statements, there are two teachings that were clearly given *ex cathedra*: the Immaculate Conception (that Mary, mother of Jesus, was born sinless) and the Assumption of Mary (that Mary at the end of her life went body and soul into heaven). Moreover, all Catholics are required to believe in it and not believing it means you are separated from the church. From my Protestant perspective, I'm struggling to understand why these two dogmas are such a big deal that all Catholics *must* believe in them. I mean, I get that the Church has taken a stance on it and that they would like everyone to believe in it. But there are other statements of faith that the Church holds as true yet doubt or disbelief doesn't expel you from the Church. I'm not seeing why these matters are so important to warrant that. It seems to me that how Mary's life began and ended is between her and God and I'm not understanding how that is relevant to me and my salvation. Why are the Immaculate Conception and the Assumption of Mary such a big deal that the popes, by speaking *ex cathedra*, require all Catholics to believe in them?