Ok not to get too complicated. So flu shots for a long time were recommended for the elderly, young, and immune compromised. I would say that rule of thumb is still good. The notion that everyone should get a flu shot is something that is new and simply pushed without really any idea what the long term consequences as far as pandemic strains are concerned. The reason for this is a concept called antigenic sin.
So your body is constantly being exposed to pathogens that it fights off without even a thought by you. Usually exposure to pathogens results in a robust immune response because you have seen said pathogen before have immunological memory B and T cells and everything gets resolved before any symptoms appear. Vaccine illicit a same strategy in which they trick the body into believing that it is being infected, usually through the use of attenuated viruses, parts of the virus etc.. coupled to a adjuvant to elicit a danger signal and the immune system kicks in, you get your immune response and you are left with memory immunity. Memory immunity is where the concept of antigenic sin comes in.
In memory you have a robust immune response that responds to specific epitopes, the response to specific epitopes is great if the virus hasn't mutated resulting in antigenic shift. The virus is cleared like I pointed out before. If the virus has shifted, you now have no immunizing B and T cells dampening the naive B cells since memory response trumps initial response. So you essentially would have an immune response to epitopes that dont neutralize the virus while having a poorer immune response to epitopes that do, poorer than you would without the memory response. This could lead to a couple things such as prolonged infection and larger immune response (one of the the theories why the 1918 flu pandemic was so bad, immune pathology).
So with the flu shots there is an idea that if you get the shot your are constantly exposing yourself to certain epitopes that over time if there is a mutated non-vaccine strain (which would be the flu most people should be afraid of and and the vaccine you do get will do nothing for) then you will have a worse immune response for clearance and a worse virus. This stuff is all based on lab studies and theories like most science and really no way to prove it. However its pretty sound science and for a healthy person exposure to a non-pandemic flu strain shouldn't be a life or death infection.