Re-sync messages between iPhone and computer after period of not syncing them? I have recently re-setup the message app on my iPhone to sync with the messages app on my MacBook. New messages are now appearing in both places, however messages that had occurred before syncing these two devices do not appear on my mac. Is there a way to have my pre-sync conversations to appear in Messages on my mac?
No. Only iMessages sent from the time you set up Messages on your Mac can be received.

Note that you are not actually syncing between Messages on iPhone and Messages on Mac - you are adding a new device to receive a copy of each message sent. When you register a device for iMessage, a set of keys (a public key and a private key) are created. The public key is sent to Apple, while the private key remains private and known only to your device. When someone wants to send you a message, their app contacts Apple and requests your public key(s). If you have four devices registered, they will receive four public keys.

At that point, their device encrypts the message once for each public key. Only your private key for a particular device can open the encrypted messages intended for that device. So if (by some technical problem) the copy of the message intended for your iPhone was sent to your Mac by mistake, it won't be able to be opened, even though it's for you and was encrypted with your public key.

Hopefully this makes it clear why messages sent prior to registering your MacBook with iMessage cannot be retrieved - simply put, they don't exist in a form that can be read by your MacBook. Even if Apple kept a copy of the messages at the time they were sent (which they for up to seven days, or until the message is received by the intended device, whichever comes first), only your iPhone can decrypt them.

This page has a good overview of iMessage encryption (which goes into more detail on what I've outlined above).