Emotions in general are detrimental to your sparring.
Usually when I have seen this said to a student it is not because we want you to get angry, but that you are sparring as if you are afraid.
If you are afraid of striking your opponent you become SLOW and hesitant.
Remember here that you should only ever spar willingly (never spar if you don't want to) and also that your partner is in the same position.
Hitting your partner (or at least trying in earnest to do so) is actually beneficial to them, if you don't hit them (by reacting calmly to the openings they present) then they will have a false sense of security/confidence going into other bouts and wind up getting hit more.
Sparring '
Angry' is usually an attempt by your opponent to intimidate you into losing confidence and not trying hard.
Many people will try this tactic, learn to deal with it and keep your calm as they are using that instead of skill to try to win.
Once you have dealt with people sparring this way a couple of times you will learn to cherish new opponents that try it as they are easy to pick apart and usually don't have any other tactic to fall back on.
As @Kristina Lex says - reacting to the situation and putting a hand or foot to any target that presents itself is key - and much easier if you aren't angry/afraid/flustered - leave emotions at the door of the Dojang or on the pads/bags .
martialarts.stackexchange.com/users/5926/Collett89
What works for your sensei might not work from you.
He is teaching you, what had worked for him.
For some, they use anger for the aggressiveness it brings.
Anger dominates certain opponents.
And in the case of draw, the more aggressive fighter that attacks more (whether contact or not) is usually declared the winner.
But anger does not work for everyone.
It results in your judgment being clouded, tunnel vision, rash decisions leading to mistakes, disqualifications due to fouls, not to mention raising your blood pressure and heart rate for the wrong reasons.
I prefer teaching students to fight with a cool head, but maintaining an 'afraid-to-lose' mentality.
As in, never let the other guy score more points than you.
And reactive fighting.
Seeing an opening results in automatic launching of a kick to that area.
martialarts.stackexchange.com/users/8980/Kristina Lex
While tigers can be considered savage in some sense, as in "uncivilized" or "fierce", they certainly don't get angry.
When they are attacking, they have a goal in mind ("kill this prey to satisfy my hunger") and they use their instinct and physical might to reach that goal, there is no emotion.
When they are defending themselves, they again use their instinct and physical abilities to save their lives, whether it's fighting back or running away.
I think that is what your sensei may have meant.
martialarts.stackexchange.com/users/9845/mustaccio
Anger is definitely not a prerequisite for sparring, specially since angry people tend to neglect their technique, but I think that you might have misinterpreted what your sensei told you.
A tiger hunts with everything it has, full concentration, full physical force and deadly precision, since they hunt completely alone.
Keeping this in mind and the fact that you yourself said that you don't like being angry, he might have ment that you should try to be less timid in sparring and go for it with more "power".
It's about focus, not anger.
martialarts.stackexchange.com/users/9842/dernobi
No, anger is not a prerequisite for sparring.
There is more than one school of thought for what your mental state should be to spar/fight.
This list is not meant to be exhaustive: Fight without emotion.
Buddhist or Daoist training for a state where thought is stopped ( mushin ) also means that your emotions do not disturb your mental state and create thoughts.
You can act decisively and without hesitation if you are mentally unencumbered.
Harness emotion for strength.
This sounds like what your instructor is suggesting.
Many athletes use music to prime themselves before competition to improve performance.
A snarling animal can be intimidating and cause an opponent to think twice before engaging.
Crazy.
Examples are Viking berserkers or Moro rebels in the Philippines.
This mental state is for killing without worrying about getting killed.
The US Army switched to using higher caliber (larger) bullets with more "stopping power" to fight the Moro rebels because they would keep fighting even after being shot.
There is a range of possibilities.
martialarts.stackexchange.com/users/5961/mattm
My sensei says while sparring you should express your inner tiger. My understanding is that I should be savage and angry. But I don't like being angry! Is it possible to be good at sparring without anger impulses?
Is being angry a prerequisite for sparring?
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<tae-kwon-do><sparring><mentality>
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"Use your inner tiger" is said to a student not because they want you to get angry, but that you are sparring as if you are afraid. If you are afraid of striking your opponent you become slow and hesitant. Remember that you should only ever spar willingly and also that your partner is in the same position. Hitting your partner is actually beneficial to them. If you don't hit them then they will have a false sense of security/confidence going into other bouts and get hit more.  Being angry is usually an attempt by your opponent to intimidate you into losing confidence and not trying hard. Many people will try this tactic, learn to deal with it and keep your calm as they are using that instead of skill to try to win. Once you have dealt with people sparring this way a couple of times you will learn to cherish new opponents that try it as they are easy to pick apart and usually don't have any other tactic to fall back on.  Anger is definitely not a prerequisite for sparring, specially since angry people tend to neglect their technique, but I think that you might have misinterpreted what your sensei told you. It's about focus, not anger. Whilst some people use anger to dominate or intimidate the opponent, it is not a pre requisite for sparring and can be detrimental, neglecting your technique.  
Anger  is not a pre requisite for sparring and can be detrimental, neglecting your technique. Some people use anger to dominate or intimidate the opponent. 