The phrase "You are annoying and immature" openly acknowledges some less favorable characteristics or behaviors in the person. This indicates an honest assessment of their flaws.
However, the statement "But you're everything to me" reveals the depth of the speaker's emotional connection and love for the person, emphasizing that despite their imperfections, they hold an irreplaceable and central place in the speaker's life.
Overall, this statement conveys the complexity of human relationships, where love and affection can coexist with imperfections and annoyances, highlighting the depth of the emotional bond between the two individuals.