This is actually my first Castlevania game ever, although I'm familiar with the basic idea. My first life was soon wasted partly because this game is a little bad about feedback - maybe it's obvious if you've played previous games, but it wasn't that clear to me that all my attacks have costs of some kind. When I press a button that moments ago Did Something and now Does Nothing, I tend to stand there in confusion pressing the button over and over again trying to figure out WTF is wrong (is the game not registering my keypress or what?) But if the problem is actually that I'm out of X, pressing the button more is not going to help, and I'm going to be very quickly overwhelmed by critters.
Now that I'm aware of the attack gauge and the separate heart cost of unions, things make more sense.
And now I can experience GoddamnedBats firsthand! (The bats are not NEARLY as annoying as the things that try to eat my head.)