The only thing that I can think could be scary about predestination is if God was not the one with the plan. To me predestination would only be scary if it was determined by the devil or by man, but since everything is predestined by our wonderful, holy and loving God, then it's not scary.
It is comforting to me, and I have learned this many times, because God's plans are so much better and greater than anything the human mind could come up with or put into action themselves. Sometimes I think I know what I want and what is best for me and then God shows up and has a different plan, that exceeds anything I could have imagined. It's also comforting because in my translation (NLT) it says, "And this gave him great pleasure." God delights in having a good and perfect plan for us and bringing it to pass. He chose us!
Why do I think Paul brings up predestination?... I think because he was talking about spiritual blessings and God adopting us, he wanted to make the point, that all of this was God's plan from the beginning. Again my translation says that even before God made the world, He chose us!
I think the Paul is praising God for it because he knows where he would be if it were not for our Lord's grace and mercy. I know full well that I have had the same moments where it hits me, that the places I would probably be right now, would not be for my best. But God... I love that phrase, but God. But God, showed up, took me under His wing, loved me unconditionally, and is currently fullfilling His purposes for my life. I'm with Paul...PRAISE YOU, JESUS!!!!