To answer the question, I would have to disagree with one premise contained in your question. I disagree that Jane found love with her cousins. Sure, she was independent, and her cousins were kind, and she was definitely welcome there, living with them. And, her cousin John even proposed, imagining her as the perfect wife for his missionary endeavors abroad. It was fairly obvious in his proposal however, that he didn't love her. She too, did not feel love for him; perhaps they loved each other in a friendly way, a companionate way, but not in the passionate and consuming way that she loved Rochester. And therein lies the answer to your question. She loves Rochester with all her heart, mind, body and soul. He haunts her; she hears his cry of despair, in a rather supernatural and strangely romantic way, calling her across the miles. If anyone has experienced this type of love, they understand that it is the most important thing, and we are willing to sacrifice a lot of things in order to keep it.
Rochester's haunting call across the moors draws her in. In leaving him in the first place, it wasn't because she didn't love him or want to spend the rest of her life with him, it was her morals. She knew he was married, and even though that marriage was at that point, in name only, Jane's scruples would not allow her to stay with him as his mistress. So she left, but could not stay away at the thought of him in pain or despair, which is what his voice relayed.
In this situation, love reigns. Love wins, above rationality, above independence, above the comfortable life that she found with her cousins. I hope that those thoughts helped; good luck!
Thank you for your answer mrs campbell, it was helpful. However, I'm not convinced that love is what really makes Jane come back. For me it may be a reason but not the only one. Actually, I wonder myself why she goes to Rochester instead of St John or even stay alone.