Tools of Characterization
In Greek mythology, Proteus is a sea god who changes his shape at will. So, it seems pretty fitting that Shakespeare's fickle character, Proteus, shares the same name because the guy falls in and out of love like some people change outfits. Proteus is also pretty good at disguising his true intentions so we could also say that his lies and deceit also make him a kind of immoral shape-shifter.
Valentine shares his name with St. Valentine, the patron saint of lovers. This seems ironic in the first scene because Valentine hates on love and makes fun of Proteus for being into Julia. The name begins to make more sense when Valentine falls for Silvia and hatches a plan to run away with her.
The name "Crab" belongs to Lance's surly natured dog, so it might be a reference to "crab apples," which are kind of sour.
Love and Romance
Ever notice the way Valentine talks about romance? It's always in an elevated, over-the-top way. In a monologue about his girlfriend, Valentine asks "What light is light if Silvia be not seen?/ What joy is joy, if Silvia be not by?" (3.1.15). Pretty poetic, wouldn't you say? We might also say that this is a little too over the top. Valentine elevates Silvia in an unrealistic way so we might question whether or not his love is genuine, especially since he basically offers her to Proteus in the final scene.
In contrast, when Lance falls in love with a girl, he explains that "She hath more qualities than a water-spaniel," before ticking off a laundry list of reasons why his new sweetie is so great: "She can fetch and carry. Why, a horse/ can do no more: nay, a horse cannot fetch, but only/ carry; therefore is she better than a jade." Plus, he says, "She can milk" a cow (3.1.6). Lance's ideas about love are practical – he's obviously interested in qualities that would make for a good wife, which, in Lance's mind, seems to be nothing more than a servant.