Why is it not advisable for piano owners to fit their own strings?
For owners of small stringed instruments like violins or guitars the strings are much thinner and are much easier to handle than piano strings. With these instruments, if you made a mistake, the worst you could do is ruin the string which would be quite easy to replace at little cost.
For piano owners, the strings are much thicker with special tools required to bend the heavier wire correctly. Using the wrong tools or techniques could very easily cause severe damage to a piano resulting in very expensive further repairs.
If you don't have the correct tools or knowledge required please contact your piano tuner.