of service to someone, be
Help someone, as in How can I be of service to you? This idiom uses service in the sense of “supplying someone's needs.” [c. 1700]
What Does It Mean To Ask Someone To Be Your “Valentine”?Here's what we know about what it means when you make someone your Valentine.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.