The discussion of love that keeps plaguing the fandom, so let me try and explain this.
Dutch and its dialect of Flemish, are a complex entities. Often resulting in different translations and interpretations, confusing the hell out of fans. The language of love isn't an exception.
Long story short, there are two different ways of saying 'I love you' in Flemish. Which are:
"Ik zie u/je graag" - literal translation: "I like seeing you".
"Ik hou van u/je" - literal translation: "I love you".
These sentences are basically the same. Some Flemish people argue that "Ik zie u graag" has less weight to it, so they use that expression for less heavy proclamations. Others think that "Ik hou van u" is just a stiff phrase and not the right fit to express what they're truly feeling inside. It depends on the person, region and context.
Understanding why Robbe told Sander "Ik zie u graag" so fast, is to understand the dating culture and proclamations of love in Belgium.
Couples in their teens or early adulthood, often met face-to-face (and to a lesser extent online). Sometimes they were hanging out as friends already, met through friends, maybe they kissed at a party...
There isn't a clear indicator as to when the actual relationship is formed. If you have kissed, hung out, went on a date, texted or just said/did something to show mutual interest, you'd pretty much consider yourself in a relationship. Unless there is, of course, a mutual agreement that what you’re doing is no such thing.
There are no 'trial periods' like weeks of 'dating'. There is: not being in a relationship and being in a relationship. Nobody really asks you to be their partner. It's just implied or stated to their family and friends. Going on a date, that's usually to test the waters or factually start / continue the relationship.
Belgium has the kind of culture that doesn't have set rules when saying 'I love you' either. If you feel it, you feel it. If you don’t, you don’t. It's not as heavy, because the Flemish use "Ik zie ... graag" a lot. For family, friends, lovers, items, … . They aren’t afraid to use it. There are people who said it after a week and meant it. There are people who had the whole “Relationship-baby-marriage” in a year. Others, ten years.
Belgians are a passionate kind of a people. Valuing friendships, family and relationships. So you could say “Ik zie u graag” very early in the relationship. As the partner, you would feel if this was meant more as an “I like you” or an “I love you”. You'd have to read between the lines...