My first go at Snape/Tonks! My first go at ever properly writing Snape actually, which is quite scary! Just to clear up something that might cause confusion: 'her' (in italics) is Lily and 'him' is Lupin. Enjoy!
7/7/2010 edit: sorted out the massive typos and errors and generally cleaned it up a bit. I know not many people have actually read this, but it was annoying me!
He should love her however she looks, but he doesn't. She's a very attractive witch and she's helped him so much and brought him out of his loneliness, but most of the time he feels nothing for her. He knows that makes him a terrible husband. He knows he should never have married her, that she deserves better than someone who can only feel anything like love for her when she looks one particular way. What's wrong with him? Most men would kill to have a partner who's a metamorphagus and can make themselves into thousands of different women. Well, he only wants one woman. And it's not her.
He loves her when she gives herself long hair, but most of the time she prefers that stupid short spikey mess on top of her head. Women should have long hair. She had long hair. She wouldn't have dreamed of cutting it off and looking like a teenage boy. Whenever he tries to get her to have long hair, she shouts at him. He doesn't mind that; when she's angry her hair turns red like a flame, just like she had. Sometimes, he even starts fights with her just to see her hair nearly ignite. Then she cries. He should hate that, but he doesn't care. He's disappointed in himself, but he squashes that feeling down. It's worth it.
He loves her when her eyes turn green. She hates green though. She says it's the Slytherin in him that makes him like it. He doesn't bother to correct her. She likes her eyes to be brown. Brown's a dull, emotionless colour, not like her beautiful, sparkling green. He used to be able to see every emotion in her emeralds. There's nothing in hers. There's not much in her green eyes either, only tears: the only way he can see them is to make her cry, but he can still see her in them.
He loves her when she doesn't wear makeup, but she insists on painting her face with all that rubbish. When he tells her he prefers her without it, they get into an argument. Her makeup runs, she cries and her eyes turn green and she gets angry and her hair turns red. She's the closest to perfect she can possibly be when she hates him. He doesn't care.
...
He's helped her through the bad times, he's stopped her feeling alone, but she can't even bear to look at him most of the time. There's no character left in him, no emotion. He used to be her teacher. He'd had personality back then, even if that version of him did make her nervous. The man she's married to now might as well be a robot! She shouldn't have married him. She hates him not showing her how he's feeling, she hates how she can't tell whether he's happy or angry or depressed, although most of the time she'd guess depressed. He was passionate and emotional. She could read him. He's gone now though and she'll just have to make do. She doesn't mind. There are some moments when he shows something, and in those moments she loves him for it. Just having those is good enough for her.
She loves him when he shouts. He comes to life. His eyes come alive and burn, his voice rises above its usual low monotonous pitch to something that could be considered music in comparison. Sometimes, in really bad arguments, little pink spots appear on his cheeks. She can start these fights easily. He hates her hair when it's not long, so she makes it extra short and sometimes she turns it pink or green if she wants him to get really angry. Admittedly, it's not the same as arguing with him; she always used to feel guilty when she had a fight with him, but in these fights she just doesn't care about making him feel upset.
She loves him when he gives her that small tender look. He used to give them to her all the time: when they were snuggled up on the sofa, when he helped her up after she tripped over something, after they were fighting. The only time she gets them now is when she's crying after their fights and even then he tries to hide them, as if he didn't mean to do it. She likes it though when she can see that small look through her tears. If she's quick, she can see all sorts of emotions in his usually blank eyes: regret, sadness, sometimes love.
She loves him when he completely loses it and they get stuck in a huge fight that sends the House Elves running away to the cellar to get away from the noise. She must look a state, but it's one of the few times when she remembers why she can love him. There's passion, excitement, fire, even fun in those moments. He's the closest to him that he can ever be when he hates her. She doesn't mind.