First of all, I don't think knowing everything about a person is at all possible. Though it's possible to believe in a misconception that you know everything :) which is quite a dangerous misconception.
There is a number of problems with the idea of "knowing everything":
1. What does it mean to know everything?
The question is a tad vague and doesn't specify what kind of knowledge is meant: will you know everything at a specific moment in time (like a snapshot of information), or do you get both snapshot and a subscription for all future updates?
2. Is it technically possible?
Knowing everything implies two parts: "gathering all the information" and then "comprehending it". Even if I imagine that somehow the first part is done (a team of a million of researchers work in parallel to acquire and dechipher *everything* about a person and save it in a super-performing cloud), I still puzzled about how I can comprehend it in finite time. Some super-computer robot that uploads the information directly into my brain?
3. Will I have enough storage in my brain?
The amount of information *about a person* is apparently larger than amount of information a person can know: it includes all person's knowledge plus more - history and all the metadata about any interaction - with people, animals, objects, history of all the thoughts, ideas and emotions. This can be substantally higher amount of information that can fit in someone else's brain, like, physically: will I have enough brain cells?
4. Is all of your information usable by someone else? Can you encode emotions and transfer them so that another person will know exactly (100%) what you felt? Some information maybe inseparable from one's body.
5. Probably the most important one: how will you clone quantum information without destroying it? Some information like "phone numbers you remember" is discrete, so it can be cloned easily. However, the way information is encoded in our physical world is quantum states, the only way to retrieve it is to "observe", observation will collapse quantum state into discrete events (so the information retrieved is largely distorted), and not only you get a distorted projection, you also destroy the original. Any observation resets quantum states. Raw ideas and emotions probably all fall into that category. So that "reading" the entire person (observing every one of it's atom and electrone) is probably an equivalent of killing them. Perhaps, the only answer is to grab a true clone from a parallel universe and then do your information retrieval experiments on them :)
Now imagine these obstacles are somehow magically overcome. Would I still prefer to know everything or not?
No, actually I'd strongly refrain. The problem is not the potential of being hurt. A much bigger problem is the potential of losing interest. All the exiting stuff in relationships is based on things you don't know about each other: it's not only discovery of a person's personality or interests, it's everyday life - exchange of new ideas, dealing with unexpected issues, pleasant surprises. Human's cognitive system is largely based on newness effect to actually see the real world, receive new information. If we have enough of familiar patterns, they work like keys to retrive image from cache. So, if you know everything about a person, you will probably stop seeing them or hearing them at all - you'll just reproduce images and voice from memory. This ends any meaningful interaction.