In the Bartimaeus Trilogy, why does Bartimaeus love Ptolemy (what happened between them)?

Okay so I'm only on the first book, The Amulet of Samarkand, but I'm so in love with the book and the characters that I'm absolutely dying to know what happened and why Bartimaeus loves Ptolemy. I know it's bad to ask for spoilers, but please just tell me what is up with Bartimaeus and Ptolemy's past! Thank you so much I appreciate it tons.