Its because Android clears optimized dex files when it needs disk-space. Hence its re-created during boot. You can more in-depth debugging  details

If you have Rooted device, you can check this app - It locks the dex files thus saves lot of time. https://play.google.com/store/apps/details?id=in.co.giis.optimisingappfixer

It's because Android clears optimized dex files when it needs disk-space. Hence it's re-created during boot. You can read more in-depth debugging details here.

If you have rooted device, you can try the following app; it locks the dex files, thus saving a lot of time:

https://play.google.com/store/apps/details?id=in.co.giis.optimisingappfixer