Nobody expects their guitar to stay in perfect tune from the first moment that they take it out of the box. Strings lose their tension over time. Hard plucking stretches them out. Atmospheric differences in temperature and humidity can alter them as well. Every once in a while, you have to perform a little preventative maintenance and tune-up in order to keep it ringing out as it once did.
The same factors of degradation that affect the strings also work upon the body of the guitar itself. Body oils and grime build up all over the instrument, particularly in places where some physical obstruction allows this gunk to collect. This includes places like the frets, the pickboard, and along the pickups, bridge, and more generally over the entire neck and body of the guitar. Keeping your guitar sounding great and preventing it from dying an early death due to neglect is the reason why you need to give it a nice treatment of natural guitar conditioner from time to time.
Yet care must be taken in the process, since one can inadvertently alter the tone or playability of the instrument if it is not done carefully. There are a number of products that will help condition the strings and clean them out so that their audio luster is restored but it is important to prevent any residue from these conditioners gravitating down to the fretboard, which also needs to be cleaned as well.
For the fretboard, which is generally the grimiest area of all, it is usually easier to make it sparkle if one removes the strings so that conditioner can be worked into all the nooks and crannies. Just as it is important to not let string conditioner onto the fretboard, it is also important to work on the neck just a portion of it at a time. Over the years, guitar necks get tensioned from the strings. If one releases all the strings at the same time, it can lead to a relaxation of the neck, which may imperceptibly flex into a new configuration and never sound quite the same ever again.
It could, perhaps, end up sounding better than before, but it is not really worth taking the chance. Take a couple strings off, clean out that area, then put them back on and move over to the next pair and so on. Always use an all natural guitar conditioner, gently wipe rather than scrub, and keep everything as dry as possible. Water is very bad for guitars.