I think it just happens. When one of my bros passed away, my mom said she often saw him in her dreams. one time she told me that when she was pregnant with another one of my brothers, she saw a dream in which they were was a coffin, and she opened it thinking it was my bro that passed away, but it was actually a new baby boy! She also told me other dreams, which i don't really care to mention, because they're sort of freakier.
I wouldn't think too much about it.