Some teammates will get angry with you if you lead trump in a game of Euchre. Despite that, leading trump is a classic winning tactic. Here's a replay of a Euchre hand that gives one example of how well it can work. You can also watch how it works in the YouTube video below.
In this example, I am left of the dealer, so I am the first person to consider the up card, the 10 of hearts. No one chooses the up card, because no one has many hearts.
After the up card is discarded, everyone is given a turn to call trump from the remaining suits. My partner chooses clubs. Since I am left of dealer, I must lead this trick by being the first to play a card.
None of my cards look like obvious winners except for one: the Jack of clubs (aka the "right bower"). It is the most powerful card, and will win the trick if played.
Some people would be afraid to play the right bower in this scenario. They might think their partner will be upset with them for taking one of their trump cards by leading trump!
However, my partner chose trump, but was not forced to do so; they could have passed. Good players will not voluntarily call trump with a poor hand. That means I can safely assume my partner has a strong hand containing several trump cards, and maybe an ace.
By leading with the right bower, I will pull trump from the opponents' hands. This weakens them, making it less likely that they can trump any ace that my partner might play.
As you can see in this example, that's exactly what happened. My partner has the ace of spades. If I had led with my 10 of spades, my left hand opponent could have trumped that trick. My partner's ace - their single non-trump card which would be expected to take a trick - would have been useless.
Fortunately, I played wisely, and led with the right bower. This stripped the lone trump card from my left-hand opponent's hand. My partner was able to collect all the remaining tricks, giving us two points.
There are no guarantees in Euchre. However, most good Euchre players will lead with trump when their partner calls trump for the reasons explained above.