Another interpretation: Why [does the Torah begin] with the letter bet? Because it is the language of blessing [berakhah]. And why not with alef? Because it is the language of cursing [arirah].
Why the Torah Begins With the Letter of Blessing
Yalkut Shimoni on Torah 2:3
דָּבָר אַחֵר, לָמָּה בְּבֵי"ת, שֶׁהוּא לְשׁוֹן בְּרָכָה, וְלָמָּה לֹא בְּאָלֶ"ף, שֶׁהוּא לְשׁוֹן אֲרִירָה.