She not only loves animals, but (also) devotes herself to their well being.

Is "also" a must in such construction of sentences? Many a times, I saw such kind of sentences without "also".

Yes, you should include it. Many a time we see many a strange thing in language use.
Thanks Mister M
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salt/pepper; ham/eggs; toast/jam; not only/but also
Thanks Philip.
I think you can get rid of "also", but not of "but" when you want to use "not only / but also". You'll also hear it without all the part "but also".
She not only loves animals, she also devotes herself...
She not only loves animals, she devotes herself...
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I thought the construction must start with "not only" as folows
Not only does she love ...