Horse or bishop?

A bishop is worth more than a knight.

You could say a knight has merits over a queen. Doesn't mean it is worth more, with implication being on average.

The bishop is just a bit better, but it is better.

In some positions yes, in others no. Any book on the subject will tell you this.

The Bishop is usually better, especially if the position is somewhat open. That's why you usually shouldn't trade Bishop for knight in the opening unless you have to. 

In some positions, a pawn is better than a rook. When people say the value of the pieces, they mean on average.

Obviously one can look at some positions and say with certainty what the value of a piece is. Sometimes you have no clue. I'd choose the bishop over the knight in such a case for sure.

The three practical ways of getting a material advantage against a decent opponent is to attack his/her king and force a defensive sacrifice, win pawn(s), by creating weaknesses perhaps, and finally by trading your knight for a bishop.

This is a general statement. Grabbing pawns while letting your king get mated is clearly a poor choice. Just like when it is obvious a knight is better than a bishop in a certain position, you don't trade your knight there.

Okay, of course the queen is better that the knight unless there's like a direct way for the knight to win the queen or game. But bishop vs knight is a real debate and there's plenty of literature on it. Basically I think it's naïve to think of the bishop as 3 and the knight 2.5 or whatever without asking yourself how it applies to the position. Taking point value too seriously I think is a common beginner mistake.

In positions where it's unclear and the bishop is active, I agree, the bishop is probably slightly better.

But in 75% of my games. My opponent chooses to exchange bishop for a horse. Why?

Probably cause it doubled your b pawns, a weakness. Usually you would develop your Bishop to b5 or g5 and retreat when attacked to keep the tension and pin on the knight. 

I can't believe none of the jokers said "a horse of course" yet lol

So, why the long face, horse.

Because your opponent is a beginner, and it seems you are too.

A horse! A horse! My kingdom for a horse.

Lol that is probably true, his opponent was playing complete novice moves.

usually this is the logic. they don't ask themselves 'should i take' they ask themselves 'is it safe to take'. 

Impossible to say. It depends which position it is.

Horse or bishop?  It doesn't have to be mutually exclusive!  During the Norman Conquest William's brother-in-law, Bishop Odo, rode into battle wielding a mace, because it was considered a bad look for a churchman to wield a sword, and he wielded his mace to great effect. 

I'd rather ride a horse than a bishop. The latter have comparatively weak backs.