When does the addition of (han)dakuten happen?

[tada]

For example
花　（はな）
生け花　（いけばな）

紙　（かみ）
手紙　（てがみ）

爆発　（ばくはつ）
出発　（しゅっぱつ）

私達　（わたしたち）
友達　（ともだち）

一箱　（いっぱこ）
二箱　（にはこ）
三箱　（さんばこ）

Is there any reason or rhyme to when this happens and when you add (han)dakuten to a kanji's reading? I know there's a name for this phenomenon too, but I forget it.

[ASHIKAGA]

They are there to make the words easier to pronounce.

Try saying TOMOTACHI, SHUHHATSU, IKEHANA, TEKAMI, etc.
Now say those words with dakutens. Easier to say them, no?

Regarding your last example, I hardly ever hear anyone say IPPAKO. It is HITOHAKO.

[Toritoribe]

I had heard an explanation that it's similar to something like "nom musculin/feminine" in French, easily-distinguishable for native speakers. However, there also exists the factor of the meaning.

e.g.
友達(tomodachi): friend
友達/友たち(tomotachi): friends

撫で方(nadekata): a way to rub
撫で肩(nadegata): sloping shoulders
*While 撫で肩 may be pronounsed as "nadekata", 撫で方 never be read as "nadegata".

I'd read a hypothesis regarding this issue in somewhere, but completely forget it.:p

EDIT:
I found wiki has an explanation.
連濁
http://ja.wikipedia.org/wiki/%E9%80%A3%E6%BF%81

[tada]

They are there to make the words easier to pronounce.
Try saying TOMOTACHI, SHUHHATSU, IKEHANA, TEKAMI, etc.
Now say those words with dakutens. Easier to say them, no?
Regarding your last example, I hardly ever hear anyone say IPPAKO. It is HITOHAKO.

edit: After a quick search, both pronunciations are given. In fact, my IME immediately gave me the correct kanji for いっぱこ but not for ひとはこ.

[Toritoribe]

edit: After a quick search, both pronunciations are given. In fact, my IME immediately gave me the correct kanji for いっぱこ but not for ひとはこ.
OMG, MS-IME(I assume you use this, right?) does it again! (again and again and again?:p)
Please do NOT put too much trust in MS-IME. 一箱 is commonly pronounced as ひとはこ, not いっぱこ. (JFYI, ATOK (http://en.wikipedia.org/wiki/ATOK) can convert ひとはこ to 一箱 correctly...)


Basically, いち(いっ), に, さん,,, is used with 漢語/音読み, and ひと, ふた, み,,, is used with 和語/訓読み.

e.g.
一通、二通、三通; いっつう、につう、さんつう
一通り、二通り、三通り; ひととおり、ふたとおり、みとおり/さんとおり(basically みとおり, but さんとおり is also acceptable recently.)

一間、二間、三間; いっけん、にけん、さんげん/さんけん
*This 間 is a unit to measure length. 一間 equalls about 180cm.
一間、二間、三間; ひとま、ふたま、みま
*This 間 is a counter used for counting number of rooms. (この家には和室が二間ある。)