Two forms of 'why?' - when to use לָמָּה and לָמָה

The basic and most common form of the word 'why?' is לָ֫מָּה with the accent on the first syllable ['lammah] and dagesh in the mem. 

Ruth 1:11 לָ֥מָּה תֵלַ֖כְנָה עִמִּ֑י

However, when it occurs before a word beginning with alef, ayin or heh, it is usually לָמָ֫ה with the accent on the second syllable [la'mah] and no dagesh in the mem. (There are some exceptions for words beginning with heh: it's not entirely consistent)

Ge 27:45 לָמָ֥ה אֶשְׁכַּ֛ל גַּם־שְׁנֵיכֶ֖ם יֹ֥ום אֶחָֽד׃

Gen 43:6 לָמָ֥ה הֲרֵעֹתֶ֖ם לִ֑י

Judg 12:3 וְלָמָ֞ה עֲלִיתֶ֥ם אֵלַ֛י הַיּ֥וֹם הַזֶּ֖ה לְהִלָּ֥חֶם בִּֽי׃

Note that before words beginning with other gutturals het and resh, it is still לָ֫מָּה.

2 Sa 24:3 לָ֥מָּה חָפֵ֖ץ בַּדָּבָ֥ר הַזֶּֽה׃

1 Sa 28:12 לָ֥מָּה רִמִּיתָ֖נִי וְאַתָּ֥ה שָׁאֽוּל׃