Overreacting to a win can cause you to become complacent.
Overreacting to a loss can cause you to lose hope and give up.
Either way, the end result will be less than optimal.
You can follow the 10-Minute-Rule:
If you win, take 10 minutes to celebrate, then get back to neutral. If you lose, take 10 minutes to grieve, then get back to neutral.
If you take a look at the world's highest-performing athletes, like Kobe Bryant, they don't tend to over-celebrate their wins, or over-dramatize their losses. Instead, they take time to celebrate or what they could have done better, then they get back into the zone and keep pushing forward.