Some plants are bad companion plants and dislike being in the same vicinity. They can actually inhibit growth and vitality, make plants prone to diseases like powdery mildew and black spot, prevent root spread and stability and cause the plants to become sickly and even die back completely.
It makes sense that there will be competition for nutrients, sun, room to grow and root space in the soil.
Plants will inhibit other plants in several ways. Sometimes it might be chemical emissions which carry to the other plant, chemicals washed down by rain to the shorter plant. It might be sent through the soil via the root system to impact on other plants' roots in the neighbourhood. The impact may be only on leaves and cause wilting and eventual decay or it might prevent germination of the seeds and result in death of the plant.
Brassicas that have passed their growth period and started to die back will inhibit the growth of other plants and stop seeds from germinating.
Eucalypts have extensive root systems, needed to hold this massive tree up, and these root compete with other plants for nutrients.
Many eucalypts also exude a chemical from their leaves and neighbouring plants do not thrive.
A bad companion plant for all other plants with similar root systems.
Strongly scented herbs and flowers can inhibit the vitality of other plants.
Lantana will kill other shrubs or trees through which it grows.
Some plants, like Rue, should not be grown in the vegetable garden at all. But Rue can be grown around compost bins and manure stockpiles as it repels flies and Japanese beetles. Rue can cause itching, so wear gloves when handling it.
Plants grown under Wattle trees seem to be susceptible to powdery mildew, black spot, mites and whitefly.
Notice what grows in the wild. If a tree does not naturally have other plants growing under its canopy then it won't in your garden either, no matter how hard you try.
An example would be oak trees and walnuts.
I have listed below the vegetables which do not grow happily in each other's company. This is just some, as I am sure you know more from your own experience.
And bear in mind what I said in the Gardening page that each country will have its own unique species of plants and the predators and insects which evolved in tandem.
Likewise, the bacterial infections which are indigenous to your area.
Bad companion plants will become obvious as you combine different plants. It is not as extensively researched as good companion planting - it is merely the opposite.
Test your own planting combinations. The listing above is a good indication of tried and tested mixtures but you will need to be mindful of your local varieties and species of plants and their pests.
Make notes and each season will show the effects of the combinations and their intensity of impact. It may be that some plants are ok together in ones or twos but in greater numbers side by side will be too much. In others even one plant in its vicinity will be enough to turn up its leaves in horror!