My father shlita (quoted in Gilyon Divrei Torah Kretchnif-Yerushalaim) explained that the Torah is teaching us that a person has to clearly recognize that whatever he has, both in terms of ruchnius and in terms of gashmius, was not acquired through his own strength or intelligence. Rather, everything is a gift from Hashem that He gave out of His pure goodness. Even if a person serves Hashem all day, he should not think that he deserves reward. He should realize that whatever he has accomplished is only a gift from Hashem.
When the Torah says ‘the man whom I shall choose’, it means: The fact that you are a man is only because Hashem chose you. For a person to reach this recognition, he needs to be humble. Thus, the verse refers to ‘the staff’ which will ‘blossom’. This indicates that a person’s humility (the phrase ‘mateh yado’ often is used to refer to anavah) must blossom in order for him to be able to realize that whatever he has is not his own doing.