Why am I so annoyed that my new neighbors keep parking in my parking spot?

I have two spaces one car, they have two cars and two spaces. There are multiple other spots all assigned for other tenants. Even though we only have one vehicle we like to keep our second one open for our guest since it is our spot. I just don't understand why they can't just use their two spaces that are assigned to them. Now they have completely taken over one of my spots so they get one and a half spaces each and we only get one. Hardly seems right considering we are the one with a vacant space not them. I know it sounds petty but it's urging me. Eventually there will be an issue when we need to use it and I don't want to have to make them move their vehicles.