Most of the effective self help advice is personal advice meets trust means relevance at that point in life.
Same book can be ridiculed and appreciated by different people, sometimes even by the same person when they are in different stages of their life (especially if they have less self-awareness).
"You already know whether you are in love or not, you just need to trust this voice."
Depending on where you are in life and who you are as a person, this statement can simultaneously be called a platitude, or a meaningful statement that someone finds allows them to decide what is important in life at that point.
"You already know" advice amplifies your existing belief. It's about taking a 60% confidence guess to a 95% confidence guess. Stated like that it becomes obvious that it's appropriate if someone vaguely suspects the correct thing, but dangerous if they vaguely suspect the wrong thing.