$requete = "SELECT * FROM Content";
$result = mysql_query ($requete,$db);
$num_rows = mysql_num_rows($result);
for ($i = 0; $i < $num_rows; $i++)
{
$seb = mysql_fetch_object($result);
echo $seb->text;
}
Basically, mysql_fetch_object() would take the current row.
Since you only call it once, it only takes the first row.
Think about it, just running a loop that only does output shouldnt magically change the contents of a variable.
Hence you have to assign to $seb to change it.
Conversely,
$requete = "SELECT * FROM Content";
$result = mysql_query ($requete,$db);
while ($seb = mysql_fetch_object($result)
{
echo $seb->text;
}