Why does my horse nudge me?

given. One particular experiment demonstrated that horses are extremely capable of determining whether or not their owner is paying attention to them or not based on cues such as eye position or body language.
Another small study even found that horses are more likely to use visual attention seeking cues when their owners eyes were open and more tactile efforts such as nudging or pawing when the owners eyes were shut or the horse felt that the owner was not looking at them. This is why ignoring a horse to reduce attention seeking behaviour doesnt always work immediately because the behaviour is already ingrained by operant conditioning where in the past the horse has learned that nudging leads to a reward.
They may be in pain
A small study that induced mild pain and discomfort on a horse to measure behavioural response showed an increase in attention seeking behaviours such as nudging pushing or mouthing in horses experiencing mild pain. This is a difficult area to study as it is hard to study pain without causing it which is something we try to avoid doing. But in general the most usual response to pain is to withdraw and avoid attention. Anecdotal evidence exists to suggest that some individual horses with medical conditions such as mild colic or lameness may become more clingy towards their owners but this would be unusual. If a horse is experiencing dental discomfort they may be nudging us to try to alleviate that pain.
They might be frustrated
In some cases horses will nudge us to get us out of the way so that they can get to where they want to be. It may also be in response to an upcoming unpleasant experience such as a dewormer that they have spotted in our hands.
What can we do about it?
If this is a new behaviour it is always worth speaking to your vet who can carry out a clinical examination of your horse including a dental exam to rule out any physical reasons like pain or itching for a change in behaviour. Horses tend to be very honest in their actions so any change in behaviour should be taken seriously.
Providing alternative resources for horses
This is always a helpful solution if we want to discourage them from a specific behaviour. If your horse constantly nudges to get a scratch and you would prefer them not to you could invest in a scratching