budha (mercury) & shani (saturn) are both sons of the luminaries (chandra/moon & surya/sun respectively) & are inimical to their fathers.

mercury is the thoughtful application & control of the mind (moon)
saturn is the thoughtful application & control of the ego (sun).

these 2 planets can put a damper on the freedom that the luminaries wish to have in just shining their light.

they wish to direct the energies of their fathers in ways that they deem useful or correct.

also note that they are the 2 gender neutral physical planets (ketu is also regarded as gender neutral but is a shadow)

also gotta love that venus is their bestie