Have to address clearly that the interpretation of Matthew 13 posted here a few days ago and the traditional interpretation are not mutually exclusive.
For example, the mustard seed can be Jesus and disciples/believers at the same time; The merchant who seeks fine pearls can be Jesus and disciples/believers at the same time as well.
But I think the two interpretations come in hierarchy: First comes the mustard seed and the merchant being Jesus, then comes the two being disciples/believers.
Simple explanation is: “disciple" means learner. Who are we learning from? Jesus. So the mustard seed and the merchant refer first to Jesus then to disciples.