Spike has always shown love and affection for Drusilla. After all, he owes her his life (or unlife). She created him so she would no longer be alone in the world. They were together a long time. He loved her with all his heart. In a sense, she created him. Without Drusilla, Spike would never have known love.
After being rejected by love as a human, as a vampire he got that missing part of himself. He got Drusilla. She completed him. They completed each other. He gained such courage from her, she was his inspiration. His reason for being.
Although it would appear that Spike was in control, this could not be further from the truth. Drusilla holds all the power in this relationship. What ever she wanted, he would get for her. Even if it was a hundred thousand miles away. Her every whim he took care of. He didn't have to, he could have left her at anytime, but he didn't. He loved her and wanted to take care of her.
Spike was the only one who really took the time to understand Drusilla, and her gift. He encouraged it, and used it numerous times to his benefit. Drusilla has predicted some of the great moments in Buffy History. You just have to listen, as Spike did.
Their love was pure, and so innocent. Spike and Drusilla were the first vampire couple that we ever saw in Buffy, bringing a more humane aspect to the vampire. Before this, vampires were seen as heartless....making it all about the blood. These two changed that image. You could see their bond in every look, every touch that they shared. You could see Spike's awe of this black beauty which he beholds. It's truely moving, and endearing. Because of Drusilla we started to see Spike as a man, rather than just another vampire. And it is because of this little slice of humanity that he has survived so long in the Buffy world.
Of course, we know that Drusilla can take care of herself...when she has to. Our Dru has never really been alone. All of her life (and her death) she had a man to look after her. Her separation from Spike made her grow up a little. She is no longer a little girl, tormented by inner demons. She is powerful and in command of herself.
Her insanity leads to misunderstanding by most of the other characters. They fear it, or don't quite know what to make of it. Angelus found it amusing, and would only listen when it suited him. He was in awe of her power, but he did not understand it.
Spike understood it, he encouraged it. He knew what she was capable of. He knew she could hypnotise people, Angelus did not.  Drusilla's relationship with her sire is one of control. He has it, and that leads to an abusive relationship. Angelus hurt her, in every possible way. Spike wants to protect her. It is that urgent need to protect Drusilla that drove Spike to the alliance with the Slayer. All he wanted was Drusilla.
Who knows what the future holds for these two? We may never see them together again. But we do know that love is eternal, and it doesn't matter who they're with, or what they're doing; deep down they will always love each other and be together in their hearts.
Spike & Drusilla