Hamlet has sworn that avenging his father takes precedence over any other concern (I.v.100-105) and so I believe all Hamlet's dealings with Ophelia from that point on are put out there as smokescreens along with other displays of "antic disposition" which goes into his endeavor to appear a little out of his rockers to the court. I like to call this whole enterprise "Operation baffles the brain" and in it I believe Hamlet is merely using Ophelia as a prop for dissimulation. Accordingly, the letters and gestures of affection is just a front he puts on. The letters are conspicuously lacking in any real emotional content as contrasted with the vivid, incarnate language (“flesh would melt” or (III.i.98-101) for example) Hamlet uses most often, and the stale superlatives makes it seem even more contrived coming from him. Inasmuch as the letters appear a little atypical coming from him, it catches the interest of the court. First he lets on that he's "love sick," then he goes on a fiery diatribe about womankind to signal, and this serve not only to distancing himself from her (dad comes first) and at the same time knows she's a decoy for Polonius to listen in on their interaction (III.i.143-44) thus using this opportunity to throw Polonius off the trail as to the cause of his "defect". You see, the love letters were truly meant for him!
Ophelia's reaction seems more to be an indication of feeling sorry for the state of this man who used to be of such great stature and adored by everyone (III.i.156-159) rather than experiencing a doleful loss of someone she loved. Also, when she later goes bonkers, there's no sign of unrequited love weighing her down in her song. Instead the burden as far as Hamlet goes is him having defiled and duffed her up. When Hamlet tells her to go to a nunnery, I believe he means a brothel where unwed mothers can deliver their babies, thereby providing with it a better life than if she were to her raise it and ...