I see that the goal of yoga is liberation from the karma and from the birth/death cycle. People say that it is the ultimate happiness forever. But wouldn't that be boring and monotonous if there is happiness forever?
Every religion rotates and evolves around a principal axis. Let's call the top of the axis the great positive and the bottom the great negative. So for example
- Christianity : evil/sin vs God
- Buddhism : duhka (suffering) vs nirvana
- Islam : rebellion vs surrender
- Taoism : disequilibrium vs equilibrium
- Hinduism : maya (illusion) vs brahman (reality)
People foolishly think it's the positive that determines the religion. Not so. Eg you become a christian by having conviction of sin even if you don't know of Jesus Christ. Likewise you are a buddhist with conviction of duhka even if Buddha's methods don't call you.
So only if getting out of the grip of maya calls you this yoga question may arise. Else not.
The term yoga has been described as meaning union, specifically union with the divine or ultimate reality, alignment with the true nature of things.
Wikipedia lists a range of aims for the practice of yoga: https://en.wikipedia.org/wiki/Yoga#Goals
What you describe 'ultimate happiness forever' is incompatible with Indian philosophy in two ways. First of all, there is no forever. Indian cosmology is cyclic, and all conditions and situations come to an end, even though the soul or atman is considered immortal. Second, concerns with happiness are about worldly desires, and the realms of rebirth. Pursuit of that only puts you on the treadmill of karma and rebirth. Inner development and alignment with the divine reality is associated not with happiness, but with bliss, presence in the full possibilities of the present moment, not limited or clouded by worldly desires.
This is a state which can help to act effectively to end suffering or dispel negative karma for oneself and others. But most importantly, it is not justified by that, it is a state or way of being valuable and worthwhile in itself, like the divine cosmic dance of Shiva (as Nataraja). It is the ultimate shift from extrinsic values to intrinsic ones, to a union of rational and aesthetic motivation based in a true understanding of the nature of being, and presence in the moment.
First of all Goal of yoga isn't ultimate happiness forever. There is no such thing as ultimate happiness. happiness is not a thing which you can have from something. Two different people can be halt as well as sad in same situation. Yoga teaches you to not get affected by surroundings or situations. Yoga says happiness is not a thing, it's already there within you while you are searching it everywhere. Happiness is just a state of mind. Yoga teaches you how to control your state of mind. Now it's up to you that you want to be happy or not.
On monotonous part, you eat daily, you do toothbrush daily and you do n number of different things on a daily basis. Why don't you feel monotonous then. Because these things are part of your life. If you don't do brush, your teeth gonna be bad. Your mouth will start smelling. Same is the thing with happiness. Without happiness your mind will go crazy.
There is a controversial link between what Hinduism is and what yoga is. To many yoga is a practice not a religion. Even some disagree with the Pantajali's vision explained in the sutras which you may follow for non-religious purposes at all not even for karma or reincarnation.
“The definition of insanity is doing the same thing over and over again, but expecting different results.” Albert Einstein
A yogi who believed in karma would tell you that reincarnating over and over again to experience ignorance and suffering (including boredom) is also insanity. Another yogi would also tell you that boredom is a creation of an undisciplined mind which is continuously craving for stimuli that is taking control over your (ksipta, mudha, and viksipta minds).
The truth is that nobody can tell you what Samadhi really is because it's something to be experienced not to be told then again you may get glimpses of it as you go along the way.