Why does my left hand feel weak?

I'm not a doctor but I do work in the medical field and see patients almost daily with similar symptoms. Given your age it is not likely that your symptoms are from a stroke or anything like that however wekness on one side (even just in your arm or hand) can be a stroke symptom. More often than not with a younger patient having these symptoms, there may be a buldging or herniated disc (yes the spelling is correct) in your neck pressing on a nerve root causing the weakness. It's pretty common and is something that can be diagnosed with a simple 20 minute MRI scan of your cervical spine. You should definately see your doctor. And as I said before, I am not a doctor and this is not professional medical advice - so take my answer for what it's worth.