Sometimes when something good comes your way, something you wanted all along but didn't work for or don't deserve at all, something you never thought of getting, but it's given to you anyways. It's hard to accept it especially if it's the first time this has happened to you and you're just not used to it. Sometimes when this happens, you try your very best to find some flaw or imperfection in it and fill yourself with doubt or just to convince yourself that it's not that good after all and keep telling yourself "This can't be right, it must be some mistake". Maybe you don't realise it but this has always been there, this thought/feeling. A very simple example would be your parents gave you a set of very expensive headphones, you've wanted it for some time but you just didn't expect to get it because it's expensive, right? You then start to wonder why would they even do this, something so nice that you don't even deserve. You just keep doubting and thinking of something negative to why they bought you the headphones just because they love you. You find it so so hard to accept the fact but it's true anyways. You get me?
I think that's how God treat us as well. We definitely do not deserve his love and grace but he just give it to us anyways because He loves us.