The reverse proxy that configured Nginx for HTTP traffic appears 403 - nginx-location

Question

I'm trying to use Nginx as a reverse proxy in and centos 6.9_64 environment where clients connects to my server (http://www.51ti.vip).
Nginx will forward all requests to other backend server. The communication is working on port 80.
However, once proxy_set_header XXXXX is set, it will appear 403 when accessed.
There is no relevant error information in /var/log/nginx/error.log.
Where's the problem?
Page 403 Forbidden
You don't have permission to access the URL on this server.
Note:
OS System: CentOS 6.9_64
Nginx version 1.10.2
Config:
/etc/nginx/nginx.conf as follows:
user root;
worker_processes auto;
error_log /var/log/nginx/error.log;
pid /var/run/nginx.pid;
include /usr/share/nginx/modules/*.conf;
events {
worker_connections 1024;
}
http {
log_format main '$remote_addr - $remote_user [$time_local] "$request" '
'$status $body_bytes_sent "$http_referer" '
'"$http_user_agent" "$http_x_forwarded_for"';
access_log /var/log/nginx/access.log main;
sendfile on;
tcp_nopush on;
tcp_nodelay on;
keepalive_timeout 65;
types_hash_max_size 2048;
include /etc/nginx/mime.types;
default_type application/octet-stream;
include /etc/nginx/conf.d/*.conf;
}
/etc/nginx/conf.d/default.conf as follows:
server {
listen 80 default_server;
server_name 47.75.249.199 "";
root /usr/share/nginx/html;
# Load configuration files for the default server block.
include /etc/nginx/default.d/*.conf;
location / {
proxy_pass http://sq.otherserver.com;
#Proxy Settings
proxy_redirect off;
#proxy_set_header Host $host;
#proxy_set_header X-Real-IP $remote_addr;
#proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
}
error_page 404 /404.html;
location = /40x.html {
}
error_page 500 502 503 504 /50x.html;
location = /50x.html {
}
}

Answer 1

The original problem was caused by "proxy_set_header Host $host",
there's no problem whith proxy_set_header X-Real-IP and proxy_set_header X-Forwarded-For.
But still don't understand why?
Nginx: when to use proxy_set_header Host $host vs $proxy_host
Module ngx_http_proxy_module