The main theme of the Toy Story Trilogy is about childhood: and the memories we create during those times. As children, our imaginations are boundless, not tied down by one single minded process, but a myriad of fanciful thoughts. Andy, is the embodiment that very essense we all shared at one point; a kid and his treasure trove of toys.
The film tell a simple, though harsh life lesson: that life is all about change. But that even with the sorrow and heartache that may come from sudden shifts in our reality, it's up to us to take those changes in stride and move forward. This core philosophy is played with in the films.
Toy Story 1 is the concept in it's basic form. With Woody being unable to accept the idea of being possibly replaced by the fancier Buzz Lightyear toy. Only through nearly being left behind, and learning to understand that friendship should be shared, does Woody come to accept Buzz and that Andy will still love him regardless of whatever new toys may come along.
Toy Story 2 then delves deeper into the concept with the character of Jessie and her tragic past with her kid; Emily. While the main plot is strictly about Woody needing to be save from an obsessive collector, the undlying theme is that while friends can grow apart, you should cherish the love and memories you shared no matter what happens.
Finally, Toy Story 3 brings us to the harshest part about change. This is characterized in many ways. Lotso is opposite of Woody, one who has refused to accept the change in his life and instead resents the "loss" of his kid (despite the clear misunderstanding that really took place). Woody on the other hand, is gradually learning to accept the fact that Andy may never play with him again, but still clings to the belief that above all else, he and the remaining toys should stay in Andy's care so that they may at least one day be played with by any children of Andy's.
In the end, Woody and the others are handed over to a new child, Bonnie. And in one last playtime, Andy bids farewell to the friends he has known for years. And in much the same way, so do we. Toys have come and gone, as have friends and other loved one. But one things remains certain: Our friendships will never die. No matter how many years go by, no matter what seperates us, true friendship; love, will last forever.
To Infinity, and Beyond.