How to spot moves like 7:dxe3

I mean is there a principle way of thinking maybe or does it come with experience

There are general considerations that can be useful : dxe3 fxe3 weakens the pawn structure around the white king, so the move needs to be looked at from a strategic point of view. Then after dxe3 you can see that all your active pieces (Qd8, Bf5, Nb4) are pointing at the d3 square so there might a thing there. But ultimately you have to see and calculate the combination dxe3 fxe3 Bxd3 cxd3 Nxd3+ Kd2 Nxb2+ winning the queen. Having the king walk into a discovered check even at the cost of material is a relatively common theme. But the general way of thinking is : White is cramped with a king stuck in the center, Black has active pieces, so you have to look at concrete ways to use those advantages.

The answer above covers everything pretty much but you have to start by asking yourself why you didn’t play it (as it’s one of the most obvious moves to at least consider, right?)

As already stated above while you could to try to explain it in terms of principles, many positions are about specifics and in such cases calculation always outweighs principles. 

Your last two moves attacked the c2 and d3 squares, so dxe3, allowing a third piece—your queen—your queen to attack d3, is the most natural move. If you don’t pursue the attack, then your knight at b4 will be driven back, and you will have lost time.

dont be too concerned about finding a general principle that addresses this position. Both players had already violated basic opening principles. White’s h3 was a poor move, neglecting development. You should respond by developing Nf6. Instead, you neglected your development by pursuing a premature attack. White did not develop/defend properly, allowing your attack to succeed.