Sakyamuni Buddha is the founder of Buddhism. Through his own practice and enlightenment, he reached the realm of liberation and spread the Buddhist concept. Ananda is the cousin of Sakyamuni Buddha. He served the Buddha, listened to his teachings and practiced with him. He knew the greatness and importance of the Buddha and was deeply saddened by his death.
In addition, Ananda didn't realize the status of Arhat grosvenor, so he gradually proved the status of Arhat grosvenor after the Buddha's nirvana. He deeply understands his ignorance and confusion, and he is more motivated to practice, and he can't accept the death of Buddha.
So when Sakyamuni Buddha died, Ananda felt extremely sad and lost and cried. This also expresses his gratitude and admiration for the Buddha and his firm belief in his own practice.