Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...f5/4. Nc3/4...fxe4/5. Nxe4/5...Nf6

Black has challenged White's unprotected e4-knight with another knight. This is a problem and an opportunity for White. In the 5...d5 lines this knight doesn't have the opportunity of gaining a tempo by swapping itself for a Black piece of equal value. Here, it does! So an obvious move here would be Why might White have second thoughts about 6.Nxf6+? After all, if White defends the knight then it's 100% going to get kicked away from e4 on the very next move with ...d5. The answer may lie in Black's options for recapturing on f6. For reasons of both development and pawn structure it looks as though Black would prefer to use the queen and not the g-pawn, but if White now indirectly threatens Black's e-pawn with then 6...d5 7.Nxf6+ Qxf6? doesn't work because 8.Qxe5+ would just pick up a pawn, so Black would have to go 7...gxf6 instead. The immediate 6.Nxf6+ could be met effectively by 6...Qxf6.
 * 6. Nxf6+, the Exchange Variation.
 * 6. Qe2,

This is excellent logic, but chess opening theory doesn't always reward logic. The more honest answer to whether 6.Qe2 is better than 6.Nxf6+ is "try them and see!" 6.Nxf6+, for one reason or another, outscores 6.Qe2 both in White wins and Black defeats.