Most adults underexpress love and appreciation to people who matter. The unexpressed version still exists but isn't received. The expressed version costs almost nothing and produces real benefit — both for the person hearing it and for the person saying it.
Who and how often
Children, partners, parents, close friends. Daily for those you live with, weekly minimum for those you don't. Specific helps more than vague ('I love how you make me laugh' beats 'love you').
Most people receiving the expressed appreciation are quietly transformed by it. The first time can feel awkward. After a few weeks it becomes natural. The relationship improves measurably.