why is it sometimes taking only "want to" and sometimes taking only "should" and its the same word chahte/chahiye
It would help to think of चाहिए (also written as चाहिये) as being its own word and not a form of the verb चाहना
चाहना and its other forms always mean 'want'.
Eg: मैं बैंगलोर जाना चाहता हूँ। - I want to go to Bangalore
वह आकाशवाणी पर गाना चाहती थी। - She wanted to sing on the radio
अगर तुम चाहो तो वहां जा सकते हो। - If you want, you can go there
On the other hand, चाहिए is 'need' or 'should'/'ought to'
Eg: मुझे बैंगलोर जाना चाहिए। - I should go to Bangalore
उसे आकाशवाणी पर गाना चाहिए।- She should sing on the radio
मुझे सौ रुपये चाहिए। - I need ₹100 (expresses a stronger sentiment than चाहता/want)