Nagging fear

I'm having the experience of things going so good that you can't help but think that something is going to come apart.  Why is this? I'm not saying that I lack faith in my current situation, because I don't, I just keep getting these fleeting thoughts that things just can't continue to go this smoothly.  I am extremely happy with my relationship, never been happier, and I say this with my whole heart.  In a few weeks we will have been together six month.  Not one red flag yet, and hopefully I never see one.  I am struggling with myself not to "think" too much about things, like weather or not that look on his face was disappointment or regret with me.  Can't stop feeling uneasy when he does things for me.  I am just so used to having things thrown in my face whenever someone does something nice for me.  I honestly feel that he would not do that to me, but still these thoughts get in my head from time to time.