Many, if not most, modern fly lines come with a factory welded loop for an easy leader connection. The problem is that if you’re using a straight shot of leader material that’s under 40lb. test, the diameter of that leader can cut into the the welded loop while fighting a big fish, trying to free a snag, or inducing any other stress that puts extreme pressure on the connection.
Solution: create a short mono butt section to act as a buffer between your welded loop and leader. I’ve been using a foot-long length of 50lb. or 60 lb. mono. Just tie a perfection loop (here’s a great tutorial from Animated Knots) at both ends, and you’re good to go! I’ve been using this system with my two-handed shooting head for over a year.