Is it because Rey just saved BB-8 from a net, and then the droid just decides to follow her about - or is it for some other reason?
Does BB-8 have any idea who Rey is prior to being rescued by her?
Just wondering as she literally tells the droid to go elsewhere.
According to the Alan Dean Foster's TFA novelization, he desperately wanted to find people (because the planet was a very unhospitable environment for him).
Contrary to much popular thought, desert worlds are not quiet at night. In the absence of light, an entirely different ecology springs to life. Moving with greater caution, BB-8 tried not to pause at each howl, every meep, the sounds of clawed feet scraping against bare rock. There were things in the vacant, wild regions of underdeveloped planets that would gladly take apart a solitary droid just to see what made it tick. Or roll, he knew. His internal gyros threatened to send him tumbling wildly at the very thought of such an encounter.
Droids such as him were not meant for unpopulated places, and he desperately desired to find others like himself. Or, failing that, even people.
It considered Rey his friend.
“Don’t follow me. You can’t come with me. I don’t want anyone with me. You understand?” More beeping, distinctly anxious this time. She grew angry. “No! And don’t ask me again. I’m not your friend. I don’t have any friends. This is Jakku. Nobody has friends here. Just fellow survivors.” Turning once more, she moved off with longer strides.
And before someone objects "why would he consider her his friend", I shall quote from @alexwlchan's favorite canon:
But from that moment on, Hermione Granger became their friend. There are some things you can't share without ending up liking each other, and knocking out a twelve-foot mountain troll is one of them. (Harry Potter and the Philosopher's Stone, CHAPTER TEN, Halloween).
What Rey just saved BB-8 from was far worse than death, it was the failure of his mission.
"Does BB8 have any idea who Rey is prior to being rescued by her?" - NO, there is no canon indication that he does.
He asks her name:
“Fine, you’re welcome.” Another beeping, which made her laugh. “Yes, there’s a lot of sand here. Beebee-Ate? Okay. Hello, Beebee-Ate. My name is Rey. No, just Rey.”
He is quite keen to deliver important information to proper people in the Resistance. Witness him taking the initiative to talk to R2-D2.
So, if he recognized Rey as someone important (like, Luke or Leia's or both's daughter), surely he would have informed the Resistance immediately upon seeing them as well.
I have no idea if BB-8 knew who Rey was when it followed her.
But there are good reasons for BB-8 to follow Rey, even if it doesn’t know who she is:
She can stand up to the locals.
When Rey rescues BB-8 from Teedo, she doesn’t get much resistance. If there are people going around capturing droids, and you're a small and fairly defenceless droid, sticking with somebody who can defend you is a smart move.
Plus puppy dog instinct, following somebody who rescued it. Rey probably seems like a pretty nice person at that point.
She has local knowledge.
I don't remember exactly what she says, but it's something like:
Town is that way, stay off Kelvin ridge, avoid the quicksand in [location] or you'll get sucked in.
We learn two things: (1) this is a hazardous environment, and (2) Rey knows where the hazards are. BB-8 may not know much about the local geography – so it would be wise to stick with somebody who does.
It's a pretty big desert.
We don't know how far BB-8 has travelled before it meets Rey, but it could be a while. Jakku seems to be a pretty big and empty desert.
If you've travelled for miles without seeing another creature, you might want to stick with the first (friendly) one you find. That comes with a much bigger chance of meeting more people.
It's also not obvious to me what BB-8 might recognise in Rey to mark her out from another friendly scavenger.
I think BB-8 is just acting on instinct here.
The film's screenplay indicates that his overt reason for requesting her assistance was that he was scared and lonely. This obviously chimes with her:
She heads off again. Finally he BEEPS to her -- something SWEET this time. That he is alone, scared, has no one else. This makes her stop. She turns and looks at him. Not liking him. But a gesture of her head says, reluctantly, "Come on." BB-8 quickly moves to her. They head off together.
REY: In the morning, you go.
[he BEEPS]
You're welcome.
