The Question and Answer section for The Wonderful Wizard of Oz is a great resource to ask questions, find answers, and discuss the novel.
Toto represents loyalty and unconditional love for Dorothy.
From the text:
After a few hours the road began to be rough, and the walking grew so difficult that the Scarecrow often stumbled over the yellow bricks, which were here very uneven. Sometimes, indeed, they were broken or missing altogether,...
I don't think she has a romantic crush on anyone. She was especially fond of Scarecrow though.