Who is happy ?

 Earlier in (5:22) Krishna has said that "Pleasures concieved in the world of senses have a beginning and end and give birth to misery (Easwaran).."

and because they have a beginning and an end but are ,in a way, addictive, they have no end till the end(death) .
Sri Sankaracharya's  explanations of desire,anger and implulse and how to identify them are very apt .

शक्नोतीहैव यः सोढुं प्राक्शरीरविमोक्षणात्।
कामक्रोधोद्भवं वेगं स युक्तः स सुखी नरः।।5.23।।

śaknōtīhaiva yaḥ sōḍhuṅ prākśarīravimōkṣaṇāt.

kāmakrōdhōdbhavaṅ vēgaṅ sa yuktaḥ sa sukhī naraḥ..5.23

English translation by Swami Sivananda

5.23 He who is able, while still here (in this world) to withstand, before the liberation from the body, the impulse born out of desire and anger -- he is a Yogi, he is a happy man. 

English translation by Swami Gambhirananda (on Sri Sankaracharya's Sanskrit Commentary) 5.23
yaḥ śaknōtī , one who can, is able to;
sōḍhuṅ, withstand;
iha eva, here itself, while alive;
prāk, before;
śarīravimōkṣaṇāt, departing from the body, till death-. Death is put as a limit because the impulse of desire and anger is certanily inevitable for a living person. For this impulse has got infinite sources. One should not relax until his death. That is the idea.
Kama, desire, is the hankering, thirst, with regard to a coveted object-of an earlier experience, and which is a source of pleasure-when it comes within the range of the senses, or is heard of or remembered.
And krodha, anger, is that repulsion one has against what are adverse to oneself and are sources of sorrow, when they are seen, heard of or remembered. That impulse (vēgaṅ) which has those desire and anger as its source (udbhava) is kāmakrōdhōdbhavaṅ vēgaṅ. The impulse arising from desire is a kind of mental agitation, and has the signs of horripilation, joyful eyes, face, etc. The impulse of anger has the signs of trembling of body, perspiration, bitting of lips, red eyes, etc. He who is able to withstand that impulse arising from desire and anger,
sah narah, that man; is
yuktah, a yogi; and
sukhi, is happy, in this world.

5:24 deal with What kind of a person, being established in Brahman, attains Brahman?