Why did Saladin, when Richard the Lionhearted was sick with a fever, send him a gift of fruit? Or when Richard's horse was killed in battle, he sent a steed to be led to his camp?
Saladin was an unusual man who tried to win the "hearts and minds" of people he conquered.
When he reclaimed Jerusalem, he ordered his men NOT to kill and plunder (in contrast even to the crusaders).
When he ruled Cairo, Egypt, he built hospitals and universities for the city, even though he had to take harsh measures against the leaders of his former enemies.
In his dealings with the Crusaders, he allowed them to "save face" by permitting Christian pilgrimages to Jerusalem, even though he controlled the city. In an era when chivalry was just developing in Europe, Saladin won the reputation of being exceptionally "chivalrous." He and Richard I each became the other's "favorite" opponent.
Assuming this truly did happen, there are a couple of reasons that might explain why. First of all, Saladin was Muslim, and one of the of the main principles of Islam is that Muslims should help those in need. Secondly, Saladin could use this as an opportunity to send men into Richard's camp and report back on the condition and size of Richard's army, thereby giving Saladin a strategic advantage.
I believe Saladin showed kindness to Richard the Lionheart not because of religion or to spy, though that is a distinct possibility, but because of Saladin's respect towards Richard, even though they were enemies. This respect of an enemy is common throughout history, though not expressed like Saladin with gifts. Julius Caesar had respect for Pompey, Hannibal for Scipio Africanus, Genghis Khan towards Jelaudin, even in world war one there was a deep respect between Australia and the Turkish.