文字列のリストから、ボタンを押すたびに一行ずつ取得

Sample2.cs

using System.Collections.Generic;
using UnityEngine;

public class Sample2 : MonoBehaviour
{
    [SerializeField] List<string> messages;
    [SerializeField] UnityEngine.UI.Text messageBox;

    private List<string>.Enumerator enumerator;

    void Start()
    {
        enumerator = messages.GetEnumerator();
        OnClick();
    }

    public void OnClick()
    {
        if( enumerator.MoveNext())
            messageBox.text = enumerator.Current;
    }
}