Person A has a crush on person B whom is their best and only friend.

Person B has no interest in A other than platonic.

Person A pines for months, giving small and seemingly endless gifts and showering B with attention to the point that their sibling/parent become worried.

Person B does eventually gain a little bit of a crush and eventually falls for A but is afraid that A won't like them back. This causes a small rift that begins to tear A apart.

Poor C has to watch them suffer.