Bow hair, no matter how good, will require periodic replacement when it no longer performs well. Extreme ranges of temperature and humidity can make hair change length, resulting in difficulties tightening or loosening the bow. It can, over time, become glazed and lose its ability to accept rosin, at which point it will feel like your bow 'always' needs rosining. Hair under tension will eventually lose its resiliency, become brittle, and break—especially in the hands of forceful players. Additionally, hair will break as a result of having been worn thin by the friction of playing. Therefore, players should expect bows to require rehairing at least once a year depending on usage and climate, many professionals' bows requiring a rehair 2-3 times a year. When a bow starts losing too much hair on 'the playing side' it can cause your bow to warp - which will then require you to have your bow recambered.
The relationship we have with stiff pegs or slipping pegs is a common problem that causes many musicians to take drastic measures. The three main reasons behind peg problems are:
Slipping pegs are common during the winter because pegs shrink when conditions are dry. In most cases, rewinding a string in the optimal manner is all that is required. If pegs continue to slip or turn unevenly, poor peg fit is likely the cause. A qualified technician can refit the pegs and solve the problem. Stiff pegs are often caused by expansion due to high humidity or a lack of peg lubricant. Lubricating the peg and/or re-positioning the string further from the peg box wall will help.