destination port changes every time

Hi,
I wrote a client/server programs,   the server listen on port 5000   and of course the client connected on that port , I check the communication via the sniffex.c program from pcap (adapting something) , I noticed that destination port ( used by the client) is not always the same and completely different from the source port.
Here is an output example:

Device: lo                    Filter _expression_: ip
Packet number 1:
       From: 127.0.0.1
         To: 127.0.0.1
   Protocol: TCP   ---->The client send a request
   Src port: 49611                  ---->The port is randomly choosed by the client ??
   Dst port: 5000

Packet number 2:
       From: 127.0.0.1
         To: 127.0.0.1
   Protocol: TCP                -----------> The answer from the server
   Src port: 5000
   Dst port: 49611

When I stopped the client and ran it again I got another Src port from the client
The question is : why the port opened by the client is not the same, and takes one randomly ??

thanks for help