I am currently a college student and part time intern at an organization, continuing my full time internship over the summer. Is it unreasonable to ask my current boss at this internship for a recommendation for an internship at another company for next summer? Would the knowledge that I don't want to work there another summer sour our working relationship?
Is there a requirement that you HAVE to produce a letter of recommendation? That's unusual - Letters of recommendation are almost never used in a "typical" working environment. Instead, you name references who put in a good word for you when contacted.
Whether the relation will go sour partially depends upon your boss's personality - You're the one who is best placed to gauge whether your boss will treat an employee's decision to leave as an act of personal disloyalty.
If your boss has that kind of personality, the relationship will sour no matter how many flowers and how much candy you send your boss's way. Your boss will be mostly an unhappy type because employees leave all the time.
Employees leaving is a fact of life in all organizations no matter how well run and how well they treat their employees. Having said that, employees that are well treated are less likely to leave on their own.
Let's assume that the boss is a rational, reasonable type. Then your boss should have no trouble understanding that you are taking off not because you don't want to work there anymore but because you want to work elsewhere and explore your career options without looking like a job hopper. You are young, and the young explore - and that's what life is about :)
Don't worry/obsess about how your boss will cope with your departure - the odds is that your boss will cope very well - you may be discomfited to find out how well - and just get someone else :) In the meantime, concentrate on doing a good job, because your doing a good job job will create some happy memories, which might come in handy should you decide to get back :)