That confidence (and, lets face it, ego) is part of what has always attracted Ianto to him since that first night they met hunting weevils. It's so utterly the opposite of who Ianto is, which is perhaps what makes it so appealing and inviting. Jack brings a lot of it out in him. He never would have pulled this stunt in the middle of a party with Lisa, for example.
It also means he loves watching what is happening now, loves Jack's cocky acceptance of that compliment as much as he loves watching those fingers press into him and the expression that sensation creates on his face.