Your doors are locked from the inside by pushing the handle forward to the lock position. There is a spring that stops this handle from just popping forward randomly. These springs tend to break over time.
To fix your problem you will need to remove the entire door latch mechanism from the door. It is composed of two parts, the latch and the remote.
The remote is the part that sits behind the inside door handle. It has a small spring in it that stops the door handle from flopping forward into the lock position.
To replace this spring you will need take apart the remote unit, which involves heating four metal tabs so you can bend them straight, separating the two halves, installing the new spring and then bending the tabs back into place.
The tabs break very easily, which is why you need to use heat before straightening them.
I have read one post where a person managed to insert this spring without separating the halves, but I've never tried it myself and all the threads on here discuss taking the remote apart.
If you do a search for 'remote spring' you will find some good threads.
Kirk