[the smile on her face grows, and she waits for him to come over, the answer to his question already there when he asks.]

You come to the beach to experience both sand and sea. If you wanted to only watch the water, you could stay at a distance. Getting wet is a risk you accept coming onto the shore, and this is the natural progression of it.

[in other words, it's a fact with no real solid reasoning behind it. it's not the most logical, but it happens to be better.]

And, here, you have company, in comparison to on dry land, where you would not.