A plant considered undesirable, unattractive, or troublesome, or growing where it is not wanted, is usually considered a weed.
Clover is beneficial to bees and to the soil. Lying on cool, soft clover watching the clouds move across a Queensland sky is one of the lingering memories of my childhood. The only downside of clover in the lawn is that, while it is in flower, you have to watch out for bees when walking barefoot. That is not enough to designate something a weed.
How and why did it change?
Clover was redefined as a weed for commercial purposes. When having a uniform couch lawn became fashionable, commercial weedkillers were sold that could target broad-leaved plants/weeds like dandelions in the lawn. Unfortunately for clover, the shape of its leaves sealed its fate. Weedkillers killed it, therefore it was a weed.
There is an important lesson in this that applies to many aspects of life. When someone or something is designated a "weed", consider the context. This is not to say that nothing is a weed, simply that it can be wise to consider options. For example...
- If a child does not learn in the classroom, does the child have a learning disability, or could it be that they don't fit the expectations of the classroom.
- If someone raises matters others would prefer hidden, are they being inappropriate or are they performing an important whistleblowing function.
- Are non-comformists weeds? Or are they the people who create new ways of doing things.
The answer to all these questions is, "It depends." The much touted "certainty" and "straight answers to simple questions" are often a poor alternative to a lively debate about issues where there is no one right or simple answer.
Is clover a weed? Not in my lawn where it feeds the soil, but perhaps when it invades my garden beds. I think that couch grass is a much bigger weed with no redeeming qualities unless your only priority is a nice, neat lawn.