And this won't work unless you show what those needs are before they fall in love.
So in your setup, make it clear what each character is lacking that the other can provide, even though they won't realize this until much later in the story. In my opinion, internal needs are much more effective than external ones. They pull a more emotional response from the reader.
However! What brings the two together in the first place is usually an external need, from one character or both, that they can assist each other with. Either willingly or forced. This is what leads them to spending more time with each other, which is necessary for them to see the qualities the other person possesses that fill their particular needs.
Does this sound formulaic? Well, it is and it isn't. We use this technique because it works, yet it doesn't get stale because you can do so much with it. You have unlimited possibilities you can mix and match with your characters.
Quick Tip: Most of the methods for romance-writing can be applied to friendship stories, including basic structural points in the character arcs such as "filling a need."
Of course, you can write romance in which the characters love each other because they're around each other so much they just can't help themselves. But romances like that have a way of feeling shallow (as do "we fell in love because it was prophesied that we would, so hey, let's just go with it" type of romances). If the characters have something solid between them that keeps them connected, makes them feel like they're incomplete without the other person, then readers will feel a deeper satisfaction when they reach the HEA ending.