The Baal Shem Tov writes that fragments of a person's soul are contained in his spoken words, so with every word he speaks, his life force is slightly diminished. However, positive and holy speech, due to its connection to nourishing spiritual energy centers, has the ability to add life force. [This is why one tends to feel energized and revitalized after speaking such noble words.]
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"Great is the glory in Your salvation (Tehillim 21) ... Performing a mitzva is like a salvation for the Divine Presence since Hahkadosh Baruch Hu is united with His Shechina through the performance of the commandments
MiPeninei Noam Elimelech Parshat Bo