Your iPhone 3GS might be showing some signs of aging. There are probably a good assortment of scratches on the screen. The case probably has various marks and dinks in it. The SIM tray is probably chipped and scuffed and the bezel has probably seen better days. The battery is probably lasting shorter and shorter. Your iPhone 3GS is tired.
Despite all of this the iPhone 3GS is still a very good phone and still has life in it. It just needs some tender loving care. It is often worth while getting it refurbished and getting it back to an ‘as new’ state. You will have your iPhone 3GS back to how it was when you first brought it and can fall back in love with it again. If you were thinking of parting with it, getting your iPhone 3GS refurbished instead. It will likely add value to the it and will certainly help you sell it quicker.