How is 'Chanda' (Intention) related to 'Tanha' (Desire)?
Chanda is a reason for Tanha. Ignorance (Avidya) is the cause for Chanda means it's the cause for Tanha. Then Tanha causes to increases the Chanda's density towards something.
Nirvana is understanding Chanda towards something is a useless thing. As nothing in this world is going to satisfy the intention (Chanda) the living beings have. That's why no one can reach ultimate satisfaction. If someone reaches ultimate satisfaction there's nothing he/she needs to do further as he/she is already satisfied. The person who doesn't want anything (as he is ultimately satisfied) is the person who obtained Nirvana.
Someone who obtained Nirvana has limitless metta (loving / kindness) towards the living beans. Due to that reason, they may try to share the path to Nirvana with others.