For instance, the easiest thing for you to do would be to simply try and gain access to their phone when they do not realize you have it. Times when they may have gone to the toilet, or simply left it on the counter top when they leave the room are perfect times for you to pick up the phone and look through the messages. Whilst this is completely unfair, it will allow you to look at some messages that you believe you should have the right to look at. If this doesn't seem feasible, however, then there is another way of getting the text messages and saving them to your computer.
If your friend or your partner saves their text messages to their SIM card, then you should purchase a SIM card reader. You will need to install the software that came with the SIM card reader onto your computer, and once that's done, plug in the sim card reader into your computer. Once your computer recognizes the device, you will need to get the SIM card. Take the back off the phone and remove the battery. You should see the SIM card. When you have the phone in question, you should take out the SIM card and put it in your sim card reader. Press the right buttons on the software that came with it and it should import all the information that it is able to find on the sim card and save it to your computer. Simply then put the SIM card back into the phone and nobody will ever know.