It takes a special kind of guy to approach strangers out of the blue, but Valancourt's just that kind of person. Actually, he's more like a golden retriever puppy: totally friendly and enthusiastic with his fair share of boundary issues.
When Emily and St. Aubert leave Valancourt the first time, he trails after them because he's lonely: "I will own, and why should I not? that I had some hope of overtaking you" (1.4.17). Let's be real—what Valancourt really wants is for Emily to pay him a little attention. He figures if he keeps popping up in random places where Emily is that she just might give him a chance.
It's actually pretty brilliant—or at least, it would be if Valancourt didn't keep wandering into dangerous situations. C'mon, everyone knows that St. Aubert is majorly paranoid about encountering banditti in the woods. Why try to surprise the guy? Staying away from trigger-happy gardeners might be a good idea while he's at it. But with all that irrepressible enthusiasm, Valancourt wants to be where the action is. And generally speaking, "Action" is Emily St. Aubert's middle name.
St. Aubert knows Valancourt's type like the back of his hand. Sure, he likes the guy—and maybe even admires him a little bit—but there's one major issue: "This young man has never been at Paris." (1.4.25). In other words, Valancourt hasn't yet faced all the temptations that young men experience when they're in a party town.
And being a puppy dog type in a party town can only lead straight to trouble, especially when flirty heiresses are all over the place. Valancourt just needs to grow up a little before he's ready to settle down with the love of his life, Miss Emily.
But even while he's partying it up in Paris, Valancourt does some decent things. Bailing out Bonnac shows his soft streak for the less fortunate. And Theresa sure seems happy that Valancourt is keeping her housed and fed, even while he's broke as a joke.
It's not often that a major character flaw is also a major strength, but we're talking Valancourt here. He lands in a Parisian jail and we still root for him to end up with his true love. So he's a little too generous with his money—what's the big deal? When he sees a poor shepherd struggling to support his family, he puts the cart before the horse:
His forlorn and melancholy look determined Valancourt at once; he threw down all the money he had […] (1.5.13)
Uh, Valancourt, how do you think you're going to get home without any cash? Money is a worldly problem that Valancourt doesn't really think about that much. He might not be the sharpest tool in the shed, but you can't fault him for his generosity.