Great espresso is the culmination of a process which starts with the right bean, passes through the roasting process and then is prepared by a skilled barista following very precise steps. It's not just pouring hot water over some coffee. It's much much more. And so many things can go wrong it's amazing when you sip a fine espresso. You just have to wonder about the complexity of it all...
The machine might be dirty or of inferior quality.
The water may be too hot or too cold.
The pressure may be too high or too low.
The coffee may be packed too lightly or too hard causing it to be over or under extracted.
The beans may be over or under roasted.
The beans may be of inferior quality or old.
The barista may be some college student working a summer job.
The water may not be pure.
The list goes on and on... So next time you drink a really good espresso think about everything that went into making that cup and your experience will will multiply!