The reason for this goes right back to Old Irish, is very complicated, and is not fully understood. However, if you want to know the underlying pattern that should make it easier to learn, here goes.
In English we sometimes have two forms
It didn't rain.
Did it rain?
Didn't it rain?
Well it's exactly the same in Gaelic. Every verb has two forms. One, called the independent form, is used for positive statements, and the other, the dependent form (because it depends on the extra words that make a question or negative) is used for the other three. You just have to learn these two forms for each tense for regular verbs, and for irregular verbs as well.
Now let's look at the future. The usual pattern is that the independent form ends in -idh, and you just remove this for the dependent form
Seinnidh mi 'I will sing'.
An seinn mi? 'Will I sing?'
But if you try to take the -idh off bidh you end up with just the letter b by itself. You obviously can't pronounce this, so it has to be bi
Bidh mi 'I will be'.
Am bi mi? 'Will I be?'
Then, on top of all that, cha causes lenition
Cha bhi mi 'I won't be'
Of course some people will prefer just to pick it up bit by bit, but if you are one of those people who likes to see some logic, here it is. D