Does it matter why it works if it works? Nobody really seems to know how any antidepressant really works. I took lamotrigine off-label for a while for anxiety, and I also found it quite effective.
I'll warn you of 3 things, though:
Lamotrigine can cause a dangerous skin reaction. This is very rare, but it's a reason why you absolutely should never increase your dose without consulting with your doctor.
It becomes less effective if you're taking hormonal birth control, to the extent that you may get withdrawal symptoms. At the time I was taking it this side effect was not as well known as I think it is now.
If the DMV learns that you're taking this (some states require you disclose medications you're taking), you may have your license suspended pending a letter from your doctor saying that you don't have seizures.