There are many "Peanuts" examples, but there are two examples in "Fairly Oddparents", too, as followed.
Timmy Turner and Tootie (she is in love with him, but he does not love her)
Timmy Turner and Trixie Tang (the opposite as the previous, he is infatuated with her, but she is not with him)
I cannot think of any more at the moment, but I am sure there are many more, and for Brian and Lois, Brian just has to learn to date his own species, that is why his relationships with women/humans never work out.
Looneytunes/Disneytoons