first of all we had this same conversation a long long time ago bu tI will give my answer again regardless
Lets take for instance Vader and Luke, Vader was the biggest baddest dude ever. However, because his son loved him unconditionally Vader got basically thrown back to the light. I guess Luke's dedication to the light side also inspired Vader's return to the light side.
Kyle Katarn is Jedi Outcast, after he thinks Jan has been killed by Dessan and Tavion he goes on a rampage and luke comments on how he is begginning to fall to the dark side. However, once Kyle discovers the woman he loves is still kicking he comes right down and loses any chance of falling to the darkside.
Then we look at Anakin, when his mom died he began to fall to the darkside.
So my theory is love lost can lead to the darkside but one showing love or being inlove can infact protect you from the darkside.