Because it's normal to trust the person you love.
So you believe them when they tell you they've never felt this way about anyone else before.
And you believe them when they say you're soulmates.
And then you start believing them when they say you're being too flirty with your male friends. And so you stop seeing them.
And then you believe them when they say your female friends aren't nice to him, and while you still see them you know there'll be a row later and you start wondering if it's worth the "price" of a quick drink after work.
And you believe them when they cry after losing their temper and say they don't understand what's happening to them, that they've never had a problem with their temper before (and so therefore you think it must be you). Because those of us in this situation generally have underlying self-esteem problems anyway.
And then you start bargaining with yourself. He's tired, you think. If we can just get to the summer holidays / get the house move out of the way / deal with this situation with his parents illness / whatever else everyday life throws at you, it will all be fine. And it'll be the way it is 90% of the time, all the time.
Only in time that 90% fine turns to 75%, then to 50%, then on and on to hardly ever.
And by then you're not seeing any of your friends, you're editing everything you say, and you've become something of an authority in the genuinely intricate skill of Keeping Him On The Right Side. He'll throw down gauntlet after gauntlet, and you'll dodge them again and again. But because you're not perfect, eventually you'll fail to dodge one, and a full-blown row will start. And you'll actually blame yourself because you'd done such a good job of defusing the other six bombs (to change metaphors), why didn't you notice that one?
And as a result of that, the first time he hits you you'll be concentrating so hard on defusing the argument you'll barely notice. And afterwards you'll tell yourself that wasn't really domestic violence, because you weren't on top form either and it was sort of your fault. Which of course, it wasn't, but by that time you're so screwed up you think it is.
Thank fuck I got out.