Q: Why after 2 months ppl change their program routine at gym?I havent changed mine since 7 months and still works for me.What happens if u always do the same exercises?Also,why after weight training ppl make some bicycle or the other machine in which u walk on it(i dont remember its name exactly in English:))for 5-10 mins before they finish?

A: You don't have to change your routine every 2 months, especially if you are a relative beginner, but it's a good idea in general. Your body 'gets used' to any routine when done for too long and the results diminish. All well-designed routines for atheletes, for example, should incorporate some sort of periodization and progression where the routine changes in volume and intensity with time. Recreational lifters can benefit from the same program design concepts to maximize strength/muscle gain and/or fat loss.

As I said, relative beginners (those with less 2 years of lifting experience) don't need to worry about this quite so much since they still have a large 'adaptation window' -- meaning they will make progress much easier than more experienced lifters. Still, it's a good habit to change up your routines every few months -- whether in volume (going from, say, 100 sets/week to 50 sets/week), intensity (going from 10-12 reps/set to failure to 6-8 reps/set to failure, for example), or just the type of exercises (such as replacing all barbell exercises with equivalent dumbbell exercises).

As to your second question, many people like to finish their workouts with a cooldown period. It's not essential, especially for weight training, but it certainly doesn't hurt anything. Another option is to finish up with stretching the muscles your worked out -- that's probably a little more useful in reducing future soreness. Or you can do both.