Why? you might scream in dismay.
Because it's dull. A character going through a whole series of event, or even just one, without meeting any sort of obstacle makes a rather uniform book. It robs you of a climax.
Every story needs a conflict. Conflicts aren't a part of happy little elf stories, because conflicts are difficult, dark and usually full of despair. I don't care if you write comedy, drama, fantasy, chick lit or YA. Somewhere along the road, your character needs to run into problem. It might be a boyfriend ditching her, a horridly embarrasingly stand-up gig, a dragon eating his arm, a friend betraying her or a bunch of hungry vampires glowing in the sunlight.
Like in real life, I need the contrast of struggle and pain to enjoy it when your character triumphs. Like relishing a bar of chocolate after exercising. Or getting an A on the stupid exam. Or cradling your newborn in your arms. Things like that doesn't come without a struggle.
Don't rob your characters - or your readers - of the satisfaction of overcoming hardships.