How can I ever know my wife loves me when there is no one definition for love?

I expect that you think that there are things you know to be true. Presumably the terms that figure in those claims are ones for which you lack definitions as well. So it doesn't seem as if the possession of definitions is necessary for knowledge. Of course, you have to know what the terms in the claim mean ; you might say, you have to know how to use them properly. But that doesn't require having explicit definitions of them at your fingertips. (Perhaps, you will want to respond, "Well, why doesn't it!?")

Would you agree that a cat or dog can love a human in the same way humans can love in a non-romantic sense?

No, I wouldn't. Humans are creatures with language and thought, and these features of our life permeate our loving. Dogs lack language and thought (in anything like the sense in which humans possess these), and so whatever it is that they are doing, it's not loving in the sense that we do this. Harder to answer is whether we love dogs in anything like the way in which we love another person.

Is it possible for one to be in love with the feeling of being in love, instead of loving the person you believe you're in love with?

A few reactions to your interesting question. We do speak of "lovingthe feeling of X" or "being in love with the feeling of Y", but surelythe sense of "loving" or "being in love with" here must be differentfrom the sense in which one loves one's mother or is in love with one'sspouse. The first seems more or less synonymous with "finding deeplypleasurable", while in the latter cases — well, I don't know exactlywhat we mean in the latter cases (Alan Soble, help!), but I do knowthat we mean something quite different from "finding deeplypleasurable" (which is of course compatible with saying that beingaround someone one's in love with is deeply pleasurable). OK, so giventhat, it's of course quite possible that John is in love with Hilaryand also finds being in that state deeply pleasurable. But that's notyour question. You're imagining a situation where John thinks he's in love with Hilary, but really isn't. People do speak as if theycan be wrong about whether they are in love: "I thought I loved him,but...