India and Nepal share a unique relationship characterized by open borders, deep cultural ties, and extensive people-to-people contact, often referred to as 'Roti-Beti' (bread and marriage) relations. However, despite these strong bonds, several irritants have emerged over time, leading to periodic strains in bilateral relations.

Main Irritants in Indo-Nepal Bilateral Relations:

1. Border Disputes: The most prominent and recent irritant involves territorial disputes over areas like Kalapani, Lipulekh, and Limpiyadhura. Nepal claims these territories as its own, citing historical maps and the Sugauli Treaty of 1816, while India asserts its sovereignty based on its own historical records and administrative control. The publication of new political maps by both countries has exacerbated these tensions. Another disputed area is Susta.
2. Perception of Indian Hegemony and Economic Dependence: Nepal, being a landlocked country, is heavily dependent on India for trade, transit, and essential supplies. This dependence often leads to a perception among some Nepali factions that India acts as a 'big brother' or hegemon, interfering in Nepal's internal affairs. Incidents like the 2015 economic blockade, which severely impacted Nepal's economy, further fueled anti-India sentiment.
3. The 1950 Treaty of Peace and Friendship: While historically beneficial, some sections in Nepal view this treaty as unequal and an infringement on their sovereignty, particularly clauses related to open borders and reciprocal treatment of citizens. They demand its revision or abrogation, whereas India generally prefers to maintain the treaty, albeit with some updates.
4. Hydropower Projects and Water Sharing: Disputes often arise over the sharing of benefits, management, and implementation of transboundary river projects (e.g., Kosi, Mahakali). Nepal sometimes feels that India benefits disproportionately from these projects or that its concerns regarding displacement and environmental impact are not adequately addressed.
5. Internal Political Dynamics in Nepal and Anti-India Sentiment: Nepal's domestic politics, often characterized by instability and the rise of nationalism, frequently see political parties using anti-India rhetoric to gain popular support. This internal dynamic often spills over into bilateral relations, creating mistrust and suspicion.
6. The China Factor: Nepal's growing engagement with China, particularly through infrastructure projects under the Belt and Road Initiative (BRI), is viewed with some apprehension by India. India perceives China's increasing strategic and economic presence in Nepal as an attempt to counterbalance its influence, adding a complex geopolitical dimension to the relationship.
7. Security Concerns: India has concerns regarding the misuse of the open border by anti-India elements, cross-border crime, human trafficking, and the potential for Nepal's territory to be used against Indian security interests.
8. Trade and Transit Issues: Despite agreements, issues such as non-tariff barriers, customs delays, and inadequate infrastructure at border points sometimes hinder smooth trade and transit, leading to frustration on both sides.

Addressing these irritants requires continuous dialogue, mutual respect, a willingness to understand each other's sensitivities, and a forward-looking approach to strengthen the unique and historically significant relationship between India and Nepal.