The setting, being so unique, is one of the main forces that brings Winston and Julia together. They both live in a highly regimented and analyzed society, where their ever move is watched, and where loyalty to the Party is the most important factor. This drives both Winston and Julia to be discontented and unhappy. The setting that they live in--a society devoid of love, happiness, freedom or independence, gives them the angst and dissatisfaction that drives them together. If Winstons was completely happy and fulfilled in his life, he wouldn't have felt the need to be with Julia, and vice-versa. Because the setting that they lived in was so awful, it made them coming together a joy and a very fulfilling thing. I was the only happy thing in their day, and that happiness kept them together.
Dialogue played a key role in their relationship, even though at first, it was mostly a physical act of rebellion against the Party. However, they have wonderful conversations that discuss the Party and how awful it is. Julia teaches Winston quite a bit about Party strategy, and through their conversation with each other, they unite and become one in purpose and feeling in many ways. Their conversations draw them closer and form a bond between them that is as fulfilling as their physical relationship, and they use that bond to bring them happiness.
Winston and Julia's unique characters also make them suited for one another. Winston is a disgruntled Party member, looking to find ways to rebel. Julia is a practiced rebel, and helps guide Winston along his own path. Their uniting character trait is a hatred for the Party. Their characterization as being people that don't fit the party mold bring them together in a joint cause. If Orwell had painted their characters any other way, their relationship wouldn't have been as feasible or fulfilling.
I hope that those thoughts helped; good luck!