Yup! More of this theory will be expounded upon in the next installment (as in...the one that will be following "Silvery Revelations").
Basically, this is my interpretation:
The patronus gets its strength from the happy thought/memory used to conjure it. There are things that make us feel so sharply happy at the time, but will years later be looked back upon as nothing more than bittersweet. Crushes, I think, would fall under this.
Tonks thinks she's in love, and even canonically we can tell that her mood is reliant upon Remus. But, she's in love with her own concept of Remus, not necessarily the man himself. Something about this concept makes her happy enough to use it as fuel for the patronus, thus resulting in its evident change.
Familial love can also alter the shape, as we likely see happen with Harry and his stag. It only took animal form after he learned about his father's animagus form, and at a time when he was thinking very strongly of his father. Thus, it made sense for Harry's patronus to be a stag, due to the association he had between that animal and his father.
I think that for those who do not use a memory/thought that's strongly associated with another person, their patronus will be a reflection of their own nature. Sort of like a glimpse of what they'd be if they became animagi.