I believe Romeo and Juliet think they died for love, but I also think that they actually died because of something else: their families' hate and a feud that had gone on for so long that no one even really seems to know what the feud is about anymore. Members of the house of Montague only know that they are supposed to hate the Capulets and vice versa.
If Tybalt had not taken offense at the fact that Romeo showed up to his family's party— something, by the way, that did not seem to bother Lord Capulet at all—then he wouldn't have challenged Romeo. Romeo would not have refused to fight him, Mercutio would not have stepped in and gotten killed, and Romeo would not have slain Tybalt. Romeo wouldn't have gotten banished, Lord Capulet would not have needed something happy to plan, and Juliet would not have been betrothed to County Paris. Juliet would not have had to fake her death, the friar wouldn't have had to write a letter to Romeo in exile, and Romeo would not have taken his own life, causing Juliet to take hers: none of this would have happened were it not for the hate between the two families.
At the same time, if Romeo did not love Juliet, then he would not have drunk poison to avoid having to live without her, and, if Juliet did not love Romeo, she would not have stabbed herself to avoid living without him. So, in a sense, they did die for love, but they died because of hate.