Babies cry. Thats how they communicate. It sucks as a mother to hear your baby cry. Especially when you are over tired, frustrated, overwhelmed, and just want to sleep. I know how tempting it is to put your baby into their bed and "teach" them how to sleep, by letting them cry and cry and cry until they finally go to sleep, and repeat until they have been taught to sleep.
But it is cruel. You might not want to think that or admit it, but it is. You are taking a tiny little baby, who trusts you. Who looks to you for everything they need, and teaching them that you cannot be trusted. That you won't be there when they need you. That they are alone and have to fend for themselves. You aren't teaching them how to sleep, you are teaching them how not to trust other people.
I've heard from several people lately that your baby will try and manipulate you. Babies honestly think that when you leave the room you are gone. They don't know that two halves make a whole. They don't know how to say their names. How can they have the cognitive ability to manipulate. It just isn't possible. Testing boundaries is NOT manipulating. To assign such an intent to an infant, shows a major misunderstanding of babies.
But think about it from your perspective as an adult. If you were alone, and tired and scared would you like to be left alone to scream yourself to sleep night after night?