I'm not trying to be mean or something but probably you have short attention span. Before you think I'm being sarcastic or mean, my friend is actually like that. He's a great guitar player but he has short attention span, sometimes he'll play the song halfway and then stop playing.
So you'd have to dedicate your attention if you want to learn the song. And I agree with Likui, you need to like the song you wanna learn otherwise you won't bother. Unless the song is very technical and you wanna achieve a certain level of technicality, but that'd also mean you want to reach a certain level by playing that song so it should lengthen your attention span.