Jamie has a bad habit of underestimating people. To be fair though- rarely do they surpass her exceedingly low bar for humanity. Joan Watson flies over it.
If she's being honest, she finds her fascinating in far more ways than she ever found Sherlock. Joan presents new challenges, manages to even catch her once. It's far harder to catch Joan- not literally of course, the woman really needs to vary up her schedule- but upon the canvas. Her painting is almost as good as a photograph, better if you value time. Yet, it's still missing something- a spark in her eye that she can't quite capture.
She supposes she'll just have to have the real thing. It's not a bad compromise- isn't a compromise at all- and she's fairly certain she can have her love within the year. It won't be easy of course, but that wouldn’t be any fun.