Joe,
When you play the piece I practiced with me or ask me to play it with your metronome it makes me sound worse.

Dear student,
Most music is meant to be played with a constant even pulse, like the one a metronome creates. At the very least, I believe being able to “grove” to a steady pulse is a skill every student should learn. “When” you play a note is possibly even more important to what note you play.

If I ask you to play with a metronome or with me it may be because I am hearing problems with the evenness with which you are playing the piece. For example, some students play notes too fast & leave space between notes where no space should exist. It is important to make every effort to correct this.

I could, simply offer a verbal suggestion such as, “work on your time.” This is typically unhelpful because chances are you don’t even realize anything is wrong with your time, & you can’t fix something if you don’t understand what’s wrong with it. So I ask you to play with something you already know is the ideal. This shows you where you should be working in a way that a million words could not.

So in short, the metronome is not actually making you sound any worse at all. It is just exposing the problem with your playing that you have not yet been trained to be aware of.

If you are struggling with your sense of time, it can be frustrating. But once you have it figured out, it will open up a lot of abilities & increase your enjoyment. It will also make you sound a lot more like you know what you’re doing to those pesky relatives that keep asking how the guitar lessons are going. Chances are they wouldn’t know the difference between an F & a G, but they do know it sounds bad when you appear to be starting & stopping constantly within one piece of music, & you know they’d love it if you figured out how to play something they could just clumsily bob their head to.