An unripe fruit will ripen faster if placed next to a ripe fruit because of cytokinin abscisic acid (aba) ethylene gibberellin

Ethylene

Ethylene is a hydrocarbon chemical emitted from most fruits that promotes ripening. When fruits are bunched together this creates more ethylene and thus ripens each fruit faster. The more fruits are bunched together, the more ethylene is produced and the faster all the fruits ripen - this is why sometimes the fruits at the bottom of a carton are much riper than at the top, because they are soaked in ethylene.