Disclaimer: J.K. Rowling owns these characters; I do not.
Miracle
The Dark Lord returning to power had once been something they'd firmly believed in (she more fervently than he), but after so many years in Azkaban, the breakout had seemed to be a miracle. Azkaban had the unique and utterly terrible ability to drain all thoughts of hope from its prisoners, so that eventually they felt there was no escape, and in fact no need even to try. One thing that it didn't dull, though, was their love for each other. It was what kept them living and strong.
Miracles did happen, and the breakout proved that. It was the best miracle that they ever could have dreamed of, being able to be free, being able to speak with joy, being able to experience pure joy, being able to make love once again.
Losing
They both fear losing the war far more than most of their companions, because they know that they'll be sent back to Azkaban, and therefore be separated again forever. If the Dark Lord is destroyed for good this time, with no hope of him coming back (and that is how they sense this second phase of the war will end if Dumbledore is triumphant), they have far more to lose than almost anyone else. Because of this fear, they dedicate themselves to the cause more than ever, throwing everything into it. Losing (both the war and each other) would be too cruel, after just having had hope and happiness.
Torture
The torture they have inflicted on others cannot compare with the torture of being separated from each other during the long years they spent in Azkaban. When away from each other, each second felt like ten years, the worst torture that could ever be inflicted upon a person.
If two people really love each other, being separated is the one thing they dread above all. This was the case with Bella and Rodolphus; their love was so strong that everything else paled before it. Unfortunately, this also made the separation that much more painful.
Devotion
Even though both are extremely devoted to the Dark Lord and his cause, their real devotion is to each other. People would think that she, especially, loves the Dark Lord above all else, but that isn't true in the slightest. Her heart belongs solely to him, and his to her, no matter what others think. They don't care what people say; they care only that they both know the extent of their love.
They've always been completely devoted to each other. They've never strayed from their marriage; none of them ever had the desire to.
School
They had met far before they went to Hogwarts, of course, as their families were friends, although not extremely close ones. But it was during the years they shared together at Hogwarts that they became extremely close, eventually leading to the relationship that was absolutely perfect for both of them – thrilling, dangerous, passionate, and above all, filled with love, however it might seem to others. Even though they think that the management of Hogwarts and some of the rules (such as letting any witch or wizard in, not just the pure-bloods) to be extremely poor, the school where they fell in love at will always have a special place in their hearts.
Decisions
Joining the Death Eaters hadn't seemed to be a decision at all for either of them, and neither was falling in love with the other. It was simply what felt right, what felt utterly perfect, but those words – no words can, really – cannot convey the burning passion that each had for the other. It was far more than lust, definitely, and more even than love – it always had been. It was something that neither could live without, something that they both needed desperately.
Decisions wasn't really a word for either of them. They lived according to their instincts, never stopping to consider the impact of what they did. It was they way that they always had been, and it was the way they always would be.
Stargazing
Stargazing was one of their favorite things to do, even though they didn't have too much time for it lately. Still, this did not dull their passion of the bright stars that shone down, giving them courage. Astronomy was by no means either of their favorite subjects when they were in school, but it still was fascinating to examine the stars above, prompting them to consider everything in the universe that they lived in, and also prompting them to cherish each other all the more because of the short time (in comparison to the heavens that shone down on them) that they had on earth.
Dangerous
Both of them have always lived lives on the dangerous side, as joining the Death Eaters showed. They both have always wanted more than they were given, and they've always craved the next thrill. Both are extremely daring, and they absolutely adore that about each other. Their relationship wouldn't work if one of them couldn't match the other in their bravery and love for thrills and risks. The sense of pure euphoria that they experience after something wild is only increased if they share the experience together.
Mystical
The way the enchanted each other seemed as if some mystical force was at work. Both could never get too much of the other. It had always been that way as long as they could remember; both were immediately entranced by the other. Both had immediately wanted to know as much as they could about the other upon meeting them, as there was something within each of them that called to the other. It did seem absolutely mystical at times, the way that they often knew exactly what the other was thinking.
Forever
No matter what happened, whether they were, they knew they would be there for each other, both as friends and lovers, their love lasting for this lifetime and however many came after it. Their love would last forever, growing more passionate and wonderful with each day, as it had done since they day they fell in love. Forever was the only span of time long enough to convey exactly how long their love would grow and last, the pure passion that they shared never fading.