It is this thing called FP(Frendship Points). These are used for the lady in Slateport and who gives your pokemon ribbons for liking you. It is also used for friendly evolutions, such as Pichu and Golbat. In particular, Golbat needs 10,000 FP to evolve. The soothe bell doubles the amount of FP recieved (Everything FP is hidden and there is no way to find out how close you are, only by going to the lady in slateport in the outsede market, left side, three tents down from the top.)
There are ways to decrease your FP, too. These ways are feeding herbs from Lavaridge or any other place you find them, fainting, and whiting out. How many FP you lose is determined on how bitter the herb is.(Revival herb most pungent, the one that cures stat problems least) The revival herb subtracts 2000 FP from whatever pokemon you gave it to. The herb that cures stat problems subtracts 500 FP.
When a pokemon faints, it loses FP. How much FP it loses is determined on how much the level difference is. If the level difference (You are lower) is 1-50, you will use 1000. If the difference is 51-98 [it can't be 100] (You are lower) you lose 500 FP. If the difference is 1-50 )you are higher) You lose 2000 FP. IF the difference is 51-98, you lose 4000 FP. Whatever pokemon faints loses the FP.
When you white out, all of the FP in your party go down by 5000. If a pokemon's FP is at 10000, it is impossible for it to lose any FP by any means.
There are two ways to gain FP -- battling and pokeblocks. Gaining FP is a long, and hard process. When a pokemon wins a battle where the opponents pokemon is a higher level than yours, you gain 10 FP. With the soothe bell, you would gain 20. If you win a battle where the pokemon is a lower level than yours, you gain 5 FP, or 10 with the soothe bell.
Pokeblocks, however, are much easier, but you will have to do each about halfway. Look at the pokemon's nature. It's nature tells you what types of pokeblocks it likes.(I'm not going to say because it would take up a lot of characters, and I'm not writing a FAQ here.) When you give a pokemon a pokeblock that it likes, it's FP goes up by 50, or 100 with the soothe bell. When you give a pokemon a pokeblock that is okay, it's FP goes up by 25, or 50 with the soothe bell. If you give a pokemon a pokeblock that it doesn't like, nothing will happen except ypu taking up pokeblock space.
Overall, the odds are against you, but that is only because every friendship evolution is worth it. Crobat is an amazing pokemon, and if I had to rate on a scale from 1-10, I would say 9. It usually takes about 2-3 days (If you are normal). You'll get it eventually, just don't white out, faint or give it herbs(potions are fine). WHEN A POKEMON REACHES 10000 FP, IT CANNOT DECREASE BY ANY NORMAL MEANS!