He came to the barn to see his pup
Lennie came to the barn to pet the puppies, which he enjoyed doing because he found them soft and comforting.
Because Lennie doesn't want to be away from his new pup.
Curley's Wife
Lennie is alone in the barn with his puppy, which is dead. Lennie still strokes it sadly, saying, "Why do you got to get killed?
Curley's wife has killed his dream through her stupidity. In his mind, she was the reason for her own death and the reason why Curley wants to kill Lennie. George will not participate in the dream without Lennie, and therefore Candy will not go to the farm at all. He will remain at the ranch and live the rest of his pessimistic life.
Lennie is hunched over because he is feeling guilty and ashamed for accidentally killing Curley's wife in the barn. He understands the consequences of his actions and is overwhelmed with remorse.
Lennie is in the barn with a small puppy in his hands that he accidentally killed by stroking it really hard!
Lennie spends time in the barn to be close to the puppies and horses, which provide him comfort and companionship. He enjoys their company because they do not judge him like others do.
So he could stay with his newborn puppy-sweet huh?
Lennie leaves Curley's wife in the barn because he accidentally killed her in a fit of panic. He takes the dead puppy to the river because he wants to give it a proper burial, as he doesn't understand the severity of his actions towards the puppy in the same way he does with Curley's wife.
George knows that Lennie will want to sleep in the barn on Friday night because Lennie always wants to tend the rabbits there, and it brings him comfort and joy. Additionally, George wants to keep Lennie away from the other ranch workers because he knows they can be mean and may potentially harm Lennie due to his disabilities.
If George and Lennie get in trouble, the plan is to meet at their designated spot by the river and wait for George to come get Lennie. They have a backup plan to flee and seek refuge at a nearby ranch where they have employment waiting for them.