I believe that there are a few reasons that grammar is such an unpopular subject to study.
1. Many of the rules are arbitrary. This means that someone somewhere decided that we would do things one way, and that's the way it is. People in general are opposed to rules that they feel are arbitrary.
2. Learning proper grammar is very difficult, more difficult than native speakers want to realize. Those who have been speaking and writing the language don't want to be told the rules. They feel that they already know them.
3. Most people don't want to have to learn the parts of an engine before they can drive a car. Why should I have to know the difference between a transitive verb and an intransitive verb?
Because all the teachers are w*nkers
Edit by DaftVirgin:
I personally hate grammar because it's just too much. The fact that you have to learn every rule of the (insert a language) grammar by heart in order to communicate, is just demotivating. In some languages grammar can also be very complicated (like French) and it gets harder if you are learning multiple languages because every language has its own grammar (trust me, I'm learning Dutch, German, French and English at school).
More thoughts
There is evidently a lot of confusion about this subject. Not least, confusion between 'grammar' and 'the rules of grammar'. Fortunately, it's demonstrably not necessary to learn 'every rule' of any language's grammar by heart in order to communicate, or none of us would ever be able to communicate in any language!
It's true that many of the rules of grammar are, or seem, arbitrary. But this certainly does not mean that 'someone somewhere decided that we would do things one way, and that's the way it is'. The norms evolved over a very long period of time, and are still evolving. All users of a language contribute to this process of evolution.
It's true that some teachers (of any subject) can be inadequate, and thus deprive their pupils of the pleasure that they should be able to take in the achievement of competence (in any subject). That is a reflection on the teachers, not the subject.
Why should you have to know the difference between a transitive verb and an intransitive verb? Well, you don't HAVE to know, obviously, but why do you object to knowing? The more you use your brain, the better it will serve you for the rest of your life.
Finally, here is another, equally valid, question to ponder:
'Why do so many people love grammar?'
"On a train" is correct grammar.
The grammar is correct as it is.
Grammar.
No. The correct grammar is; "Yes, I have a girlfriend."
Can you end my day with correct grammar
No, "will be had" is not a correct grammar. The correct grammar would be "will have."
No, it is correct grammar, not a correct grammar.
The correct grammar is: "Are those correct?"
"On a train" is correct grammar.
The correct grammar for this sentence is: "When did you send it?"
No, the correct grammar would be "I hate you the way you hate me."
no_____If the sentence is You do do that (meaning You are in the habit of doing that) the grammar is perfectly correct and the sentence 'does have correct grammar'.
No, the correct grammar would be: "July has just started."
its has he kissed you yet that's the correct grammar-Erendiraa
The correct way to phrase this question is: "Is my mother and I correct grammar?"
Grammar.
This will be an exciting month is correct grammar.