If you really believed that the envelopes were alive and were friends then that would be a delusion. People who have Schizophrenia and people with Manic Depression can have delusions as symptoms. People who have severe Major Depression can also have delusions.
If this is a serious problem you should see a doctor to get checked out. If you are just having a little fun at work and pretending the envelopes are alive so that your job is more interesting, then that is not because of a mental illness, it is because of a boring job.
However, if the latter is the case, then you need to be careful about this because if you continue making up things like this, your mind can get in the habit of doing it. It is generally the best thing to stay in the real world even when bored and do something else to make your job less boring.
I used to work as a Registered Nurse in Psychiatry and personal experience.