I think it's important to distinguish between it being simply a hobby and something else. Which is it?
I think of a hobby as being a way to pass the time when the TV is broken (that'd be nice, what a waste of bandwidth TV is!) or it's raining and you're stuck with somebody else's children, or something.
For me, engineering as a whole is a way of life. My goal in life is to understand more than I know currently, hence why I'm here on this forum, hence why I watch Dave's videos, etc. This, as opposed to building something with a rough understanding of the bingo bullshit terms suffice to get by to achieve a larger goal. What I mean by this is, say, building a low voltage cutoff circuit for something battery powered, for example, where the understanding of how the circuit works can be glossed over as long as it works when implemented. I'd consider this to be more on the "hobby" end of the scale.
If engineering (whatever the discipline) is a way of life, it can become a bit more difficult for your partner to understand what makes you tick and why it does so, if they ever get it at all. They probably won't unless they're interested themselves. In all honesty, I got extremely lucky, however it did take time for my other half to at least half understand the "what". She almost definitely doesn't understand why it makes me tick, but she knows the order of magnitude of the numbers in the equation, if that makes sense. She thankfully understands that it doesn't take a prescribed amount of hours per week, nor a pre-arranged day or days, i.e. every Tuesday. If I want to go and work on something on Wednesday night until 3AM, she's fine with it. I also understand (this was a learning curve for me) that I can't be a complete knob, take the piss and do it every day claiming that I need to do it or else I'd become a more horrible person. That's where your end of the bargain comes in.
I have found that they also need quite an admirable level of patience as you tend to fall into the trap of not holding back when people ask how something works - you can't help it, you want to tell people how it works and accurately. They're not prepared for it and just switch off, or assume that you're trying to make them feel stupid.