I think the difference is that you are ferreting out real possibilities for future problems for your clients, they have an actual "thing" that can go wrong, machinery can break or they can get sued, etc. whereas in relationships, there's no "thing", no discrete problem, it's wholly open-ended and you are perhaps trying to close it off some so you can contain or control it.

I was interested in when you said you try to distract yourself when you find yourself thinking up future scenarios; why not just practice thinking up good scenarios instead of bad ones? If you are going to live in the future, you might as well make it a good one? No, you don't have to picture marrying a guy and having 2.5 gorgeous children by him :-) but I would take the actual (like you do with your clients) of what you know of him (Is he honest? Does he seem sincere? Are you comfortable being and talking with him?) and project that out or, if there are negatives, I'd start thinking of them "now" instead of waiting until they bloom? If you catch him lying, picturing him breaking your heart later is not useful? Practice watching for current problems you identify and talking about them.

Yes, when you are talking with clients about what material to make an item out of and need to know the relative strengths and weaknesses down the road (do you want the roof that lasts 15 years or the more expensive one that lasts 20?) there isn't a roof yet so it is natural to look at the future-roof but a relationship that is already in progress has things going on now and can be steered better by dealing with the problems now.