There are many old sayings in euchre. Don't trump your partner's ace. Count on your partner for one trick. Take a trick whenever you can. And there are many others. Most of these give sound advice to the novice player. However, there is one saying that's patently wrong: "Don't go alone on 8". This refers to making a lone call when your team has 8 points. After all, why go alone when taking your partner along will surely help you win all five tricks and earn the necessary two points for the win? Right? Actually, there are many hands where not going alone on 8 is not just wrong, but it will cost you a point and possibly the game. Most skilled players know this. Yet many inexperienced players will never play alone in this situation, insisting that it is wrong, rude or maybe just because they were told not to.
This screenshot is from a game where the score is 8 to 9, in favor of the east-west team. The dealer is in the east position.
You are sitting in the South position and order the 9 of diamonds into the dealer's hand. The fact that your partner holds the ace of clubs does not help your hand, in fact, it hurts it. If you take your partner along you will only make one point. This is because the ace will take the lead away from you. This will allow them to take a trick. That would make the score 9 to 9. Yes, you will have the next deal, but as this hand shows, having the deal doesn't always guarantee you'll make a point. By going alone, it assures you of the win.
You are the dealer and are in the South Position. You have just the turned up the jack of spades, and naturally you pick it up. Again, the score is 9 to 8, in favor of east-west team. West leads the nine of diamonds. Your partner(N), thinking this is his only chance to help out, trumps in with left. As hearts are the only suit left in his hand, he has no choice but to lead one. You play your ace, but it is trumped by West. Both of your aces were wasted and the chances of winning this game have been greatly reduced.
The jack of clubs is turned up, everyone passes and the dealer(E) picks it up, discarding the king of spades.
South starts the hand by leading the ten of hearts. West seat (the dealer's partner) trumps in with king. North follows suit with the 9 and the maker's(E) ace is wasted.
Now West seat leads the ace of spades, North seat follows suit with the 9. Now no matter what the maker does, this becomes a one point hand.
By the way, I do realize it's a no-brainer that this hand should have been played alone. Still, it does show that there are times when a partner can unintentionally hurt your chances to take all the tricks.
Remember This: Anytime you hold a strong hand and your partner cannot significantly improve it, you need to play it alone. The score should not be a factor.