Why People touch their nose when they talk to someone ?

I am not sure if nose touching is a scientifically-validated signature of deception or internal conflict. For the purposes of this question I would assume that it's fairly common knowledge in psychology that people touch nose or cover the mouth when saying something part of them does not believe to be true.
When a person lies, blood vessels in their nose may dilate, making the nose swell or appear redder. The nasal engorgement then causes mast cells to release histamine, which makes the nose itch and so may lead to the person touching or scratching it (this is probably the basis of the Pinocchio story).
Rubbing the finger alongside the nose can indicate disagreement. It may also be a semi-suppressed nose-scratch related to lying.

Pinching the bridge of the nose can show the person is evaluating something, usually negatively and with some frustration.

Sometime touching your nose can also be seen as a natural human reaction but behavioural studies show that touching your nose when you speak means you are subconsciously hiding your mouth with your hand so the person you are speaking to will not physically see that you are lying to them or deceiving them in some way! it also relates to chatting up a girl if perhaps a guy is shy.

The nose touch is probably one of the more recognised lying gestures, this gesture should be read in clusters and in context, someone may simply have an itchy nose caused by a cold or hay-fever.  A person may touch their nose several times whilst talking, or they may simply touch it once in a quick, imperceptible manner. The reason that people may touch their nose when lying is because when we lie we begin to feel more stressed, particularly if that person is lying to get themselves out of trouble. This causes blood pressure to rise which can cause the nerves in the nose to tingle. This gesture is often seen in adults more than children, and is thought to develop after the “Mouth Cover” gesture because it’s much more subtle.
Bill Clinton was shown to touch his nose almost triple what he usually did when he lied.

As you already read, rubbing the nose can be a response to an actual itch or sense of discomfort. But it can also be a sign that someone’s not being honest with you.
You can tell the difference between a “normal” nose itch, and a nose touch that is from the signs of lying – the simple itch will be satisfied with rubbing or scratching, but won’t be satisfied with just a light touch, and this is what happens when the person is lying.

Once again, if the speaker is the one who displays the nose touch gesture, it means he could be lying. If it is the listener who does it, then he could simply doubt what he is hearing.

* I personally think that if a person who you talk to is touching his/her nose it could mean that he/she is trying to say that you have something on or in your nose .

Note : Japanese do not have this kind of gestures, but for them for pointing their own nose with a forefinger, signifying ‘It’s me.’

As a conclusion mainly people believe and think that when any body touch his nose when talking it means s/he is lying or s/he is not confident enough of what he talks about.