Why do you have sadness in the first Jhana?
This is an interesting question, because the first jhana explicitly involves seclusion from the five hindrances, one of which relates to domanassa (translated in your quotation as "sadness"). According to the abhidhamma, domanassa can only arise in anger-based minds, which one might think would be totally absent in the first jhana. The commentary to SN 48.40 blames vitakka and vicaara explicitly for the potential arising of domanassa:


  nānāvajjaneyeva ca dutiyajjhānupacāre pahīnassa domanassindriyassa yasmā etaṃ vitakkavicārapaccayepi kāyakilamathe cittupaghāte ca sati uppajjati, vitakkavicārābhāve neva uppajjati.


My Pali is rusty, but this passage basically means that the bodily distress and mental harm which would arise based on vitakka and vicaara do not arise with the non-existence of vitakka and vicaara.

This gives some insight into the nature of vitakka and vicaara as per the texts; they allow for some instability by which the mind still engages in perceptions leading to domanassa, even though the mind is otherwise fixed and focused on a single object (ekaggata).