Syaoran has to be with Sakura because she completes him. Sakura loves people and she loves life, and she has such a beautiful outlook on life that allows Syaoran to give up his serious and rugged way. If it weren't for Sakura, we wouldn't be able to see the real Syaoran. She is truly the only one that can allow him to let his guard down, and also enjoy life.
Also, Sakura needs Syaoran. He can love her in such a way that she is able to further perservere. Take the final capture of the Void Card. If it wasn't for her love for Syaoran, she wouldn't have been able to capture it. I think that if there was no Syaoran, she probably wouldn't have been able to catch it, and even if she did, all her feelings would be gone. She wouldn't be the kawaii genki girl we all know and love. This is why Syaoran cannot be with anyone else but Sakura.