It does depend on the other person, but my wife knew I was into cars/mods when we were dating and it never really got in the way. Married for 6 mo. now and convinced her we should get an E90 for her Civic lease exp. We bought it and, while she doesn't get the need to add to it, she does understand that for my Valentine's/bday/xmas etc gifts, it's basically car stuff I want b/c I have pretty much everything else. She buys me that stuff for those occasions, otherwise I'm on my own and she will still occasionally say X mod is pointless or unnecessary.
I think she secretly likes the hobby b/c she gets to drive the car almost as much as me and I know she doesn't mind having the extra power from something like a jb4 in it.